concurring.
I join the opinion and judgment of the Court with the following observations.
As long ago as 1927, in Eastman Kodak Co. of N. Y. v. Southern Photo Materials Co., 273 U. S. 359, the Court *143recognized that participation in an unlawful course of conduct would not bar recovery where the defendant’s superior bargaining power led to plaintiff’s participation in the unlawful arrangement. In Kiefer-Stewart Co. v. Joseph E. Seagram & Sons, Inc., 340 U. S. 211 (1951), where plaintiff was said to have participated in an illegal scheme other than the one charged in his complaint, the Court made it clear that a plaintiff’s own delinquency under the antitrust laws would not always bar his treble-damage suit. See also Bales v. Kansas City Star Co., 336 F. 2d 439, 444 (C. A. 8th Cir. 1964); Jewel Tea Co. v. Local Unions, 274 F. 2d 217, 223 (C. A. 7th Cir.), cert. denied, 362 U. S. 936 (1960). These cases are enough to warrant reversal in this case, once it is concluded that the illegal arrangement in which petitioners participated was thrust on them by respondents. This is the conclusion reached by the Court and I agree with it.
I also agree that the in pari delicto defense in its historic formulation is not a useful concept for sorting out those situations in which the plaintiff might be barred because of his own conduct from those in which he may have been a party to an illegal venture but is still entitled to damages from other participants. Judgments like these would be better made by hewing closer to the aims and purposes of § 4 of the Clayton Act, 38 Stat. 731, 15 U. S. C. § 15, which gives treble-damage recovery to the private plaintiff injured by conduct which violates the antitrust laws.
Under § 4, plaintiff must show not only that the defendant violated the antitrust laws but that his conduct caused the damages alleged in the complaint. Normally, it would be enough with respect to causation if the defendant “materially contributed” to plaintiff’s injury, Continental Ore Co. v. Union Carbide & Carbon Corp., 370 U. S. 690, 702 (1962); or “substantially *144contributed, notwithstanding other factors contributed also," Momand v. Universal Film, Exchanges, Inc., 172 F. 2d 37, 43 (C. A. 1st Cir. 1948), cert. denied, 336 U. S. 967 (1949). The plaintiff need not show that the illegality was a more substantial cause than any other. Haverhill Gazette Co. v. Union Leader Corp., 333 F. 2d 798, 805-806 (C. A. 1st Cir.), cert. denied, 379 U. S. 931 (1964).
Under this rule, a third party proving an illegal undertaking between two defendants may recover for all damages caused by the combination. Those damages normally may be had from either or both defendants without regard to their relative responsibility for originating the combination or their different roles in effectuating its ends. This is because neither defendant, if he acted alone, could be charged with the violation; some degree of participation by both is essential to create a combination within the reach of § 1 of the Sherman Act. ' Either defendant is therefore deemed to have been a material cause of the damages, sufficient to permit a third party to recover.
This may be the result required under § 4 when conspirators are sued by an injured outsider. But what is the situation when one party to the combination sues the other? Assume three situations: first, A, a manufacturer, sells to B, a retailer. A, over B’s objection, insists on B’s adhering to specified resale prices. B agrees since A’s product is an important part of his business and he can get it nowhere else. B suffers a decline in business because of an inability to match or better the price for competing products. B sues A. He is obviously in a position to prove that A was a substantial cause of his injury.
Second, suppose that when B maintains the suggested prices on A’s product, he simply sells more of C’s competing product, which he also handles. B is not hurt, but A is. A sues B.
*145Third, suppose that D and E, competitors, combine to fix higher prices. D’s best customer sets up his own source of supply to D’s great damage. D sues E, claiming that E was a substantial cause of his injury.
It is arguable that in each supposed situation recovery should be denied because the plaintiff was a party to the illegality and wrongdoers should be left where they are found. In terms of the deterrent aims of the statute permitting injured plaintiffs to recover treble damages, however, this undiscriminating approach makes little sense. When those with market power and leverage persuade, coerce, or influence others to cooperate in an illegal combination to their damage, allowing recovery to the latter is wholly consistent with the purpose of § 4, since it will deter those most likely to be responsible for organizing forbidden schemes. The principles of Eastman Kodak Co. of N. Y. v. Southern Photo Materials Co., supra, clearly permit recovery by the less responsible, but injured, party. In the first hypothetical case, therefore, B should recover from A in order to deter A and others like him from imposing resale price maintenance schemes on their customers.
In the second case, where manufacturer A, contrary to his expectations, was injured and retailer B was not, there is no reason, based on the deterrent purposes of § 4, to permit recovery from B, even though his cooperation was essential to the combination and even though had a third party been injured he could have recovered from either A or B, or from both. A, the moving force, should not be rewarded for his efforts to further an unlawful price arrangement and in effect to take from B the profits, trebled, that B made by selling the products of A’s competitor. B was unwilling to enter the illegal scheme, was motivated principally by what he thought was economic necessity — the need to avoid losing business by being unable to offer a major *146product line — and would have been only marginally deterred by the prospect of antitrust liability.
In the third case, where D and E are competitors, if D simply proves the agreement and the resulting loss, should he recover from E, absent some believable showing that E was the more responsible for the illegal scheme? No doubt E was a substantial factor in the combination and hence in the injury; a judgment for damages might deter him and others from violating the law. But D is equally responsible for his own damages. To permit him a recovery may be a counter-deterrent. By assuring him illegal profits if the agreement in restraint of trade succeeds, and treble damages if it fails, it may encourage what the Act was designed to prevent. In this situation, it is doubtful that the ends of § 4 would be measurably served by permitting D’s recovery. If judge or jury finds the parties equally responsible for the conduct which caused injury, D’s recovery under § 4 should be denied for failure of proof that E was the more substantial cause of the injury.
No simple formula can encompass the infinite variety of possible situations. Generally speaking, however, I would deny recovery where plaintiff and defendant bear substantially equal responsibility for injury resulting to one of them but permit recovery in favor of the one less responsible where one is more responsible than the other. This rule would simply pose the issue of causation in particularized form. There will be little mystery as to what evidence would be relevant proof: facts as to the relative responsibility for originating, negotiating, and implementing the scheme; evidence as to who might reasonably have been expected to benefit from the provision or conduct making the scheme illegal under § 1; proof of whether one party attempted to terminate the arrangement and encountered resistance or counter-measures from the other; facts showing *147who ultimately profited or suffered from the arrangement.
As I view the record in the case before us, the evidence is insufficient to show that petitioners were as responsible as respondents, of more so, for the admittedly illegal scheme. The evidence before us does not suggest that petitioners were equal partners with respondents with respect to the origin and implementation of this scheme for distributing respondents’ mufflers, or in terms of benefits from the scheme. In such circumstances summary judgment for respondents was improper.