We must here decide the validity and effect of a rule of the Iowa Real Estate Commission requiring all real estate listing agreements to be in a writing containing all essential terms. The trial court awarded plaintiff Donald R. Milholin judgment for a $3360 real estate commission pursuant to an alleged oral listing agreement with defendant Gerald Vorhies. In doing so, the court rejected the applicability of the commission rule on the ground of absence of commission authority to alter the common law. We hold that the rule is valid and precluded enforcement of the alleged oral listing. Therefore we reverse the trial court.
The rule requiring listing agreements to be written is 700 I.A.C. § 1.23, which became effective July 1, 1975:
All listing agreements shall be in writing, properly identifying the property and containing all the terms and conditions under which the property is to be sold, including the price, the commission to be paid, the signatures of all parties concerned and a definite expiration date.
Plaintiff contended and the trial court agreed that the validity of this rule is limited to its use as a basis for licensee discipline. This conclusion was based on the premise that the commission lacked authority to adopt a rule abrogating the efficacy of oral listings. Although the trial court noted that a manual published by the commission contained a statement inconsistent with the rule, no showing was made that plaintiff relied on the misstatement.
This court has previously recognized the enforceability at common law of oral brokerage agreements. See, e.g., McHugh v. Johnson, 268 N.W.2d 225, 227 (Iowa 1978). The court has also recognized, however, that a rule within the authority of an agency to adopt has the force of statute. Davenport Community School District v. Iowa Civil Rights Commission, 277 N.W.2d 907, 909 (Iowa 1979). Because it is clear that the common law can be changed by statute, Iowa Civil Liberties Union v. Critelli, 244 N.W.2d 564, 568 (Iowa 1976), the crucial issue here is not whether the rule would supplant the common law but whether it was within the statutory authority of the agency.
Principles governing our review of that question are discussed in Hiserote Homes, Inc. v. Riedemann, 277 N.W.2d 911, *554913 (Iowa 1979), and Davenport Community School District, 277 N.W.2d at 909-10. The rule is presumed to be valid, and the burden is on the person challenging it to demonstrate that a “rational agency” could not conclude the rule was within its delegated authority.
Applying that standard here, we find plaintiff did not carry his burden. Code chapter 117 is a regulatory law and not merely a licensing act for raising revenue. Pound v. Brown, 258 Iowa 994, 998, 140 N.W.2d 183, 186 (1966). It establishes the real estate commission and vests it with far-reaching authority to license, regulate and discipline brokers and salespersons. For example, licenses are to be suspended or revoked for practices “harmful or detrimental to the public.” § 117.29(3). The commission is charged with investigative and disciplinary responsibilities relating to various kinds of specified misconduct including being unworthy or incompetent to act as a real estate broker or salesperson “in such manner as to safeguard the interests of the public.” § 117.34(8). The commission is also granted express authority “to promulgate rules to carry out and administer the provisions of [chapter 117] consistent therewith.” § 117.9.
The overriding purpose of the statute and its delegation of authority to the commission is to protect the public. Cf. Red Carpet-Barry & Associates, Inc. v. Apex Associates, Inc., 130 Ariz. 302, 635 P.2d 1224, 1226 (1981) (“The purpose of the statutes is to regulate the conduct of real estate activities so the public may be protected.”).
The rule requiring listings to be written is analogous to a statute of frauds. See Restatement (Second) of Contracts, Statutory Note at 285, § 110(5), and Comment b to § 126 (1979). In discussing a rule containing almost identical language, the Vermont Supreme Court said:
Ostensibly, the purpose of this rule is for the protection of the public to establish fair dealings between parties, standardize the procedure and practices in the real estate business and to prevent fraud. Its purpose is similar to that of the statute of frauds, which, ... “is to prevent a party from being compelled, by oral and perhaps false testimony to be held responsible for a contract he claims he never made.”
Green Mountain Realty, Inc. v. Fish, 133 Vt. 296, 299, 336 A.2d 187, 189 (1975). The real estate commission could reasonably believe that its rule provides similar protection to the public. See Gaudio, The Iowa Law of Real Estate Brokerage, 30 Drake L.Rev. 437, 461 (1980-81). The legislature has vested the commission with broad authority to determine what practices are harmful or detrimental and what constitutes unworthiness. See Miller v. Iowa State Real Estate Commission, 274 N.W.2d 288 (Iowa 1979). The rule thus implements the commission’s statutory duty to safeguard the public and provides guidance for brokers.
The Kansas Supreme Court voided a similar rule in Marcotte Realty & Auction, Inc. v. Schumacher, 225 Kan. 193, 589 P.2d 570 (1979). The court characterized the Kansas statute as merely a licensing act, however, and did not apply a rational agency standard. We take a broader view of the commission’s responsibility under the Iowa statute and apply the rational agency standard in testing the validity of the rule. The commission is charged with regulating broker conduct to protect the public, and the rule is a rational way for it to do so. We conclude that the rule is within the commission’s authority.
The rule’s effect is not to invalidate oral listing agreements but to make them unenforceable upon proper objection. See Red Carpet-Barry & Associates, Inc., 635 P.2d at 1226; Green Mountain, 336 A.2d at 189-90. In view of defendant’s affirmative defense relying on the rule, we hold that the trial court erred in enforcing the alleged oral listing agreement. No alternative basis for recovery has been urged, and therefore we do not consider whether any such basis exists. See, generally, Restatement (Second) of Agency § 468 at 399, Comment on Subsection (2) (1958); Restatement (Second) of Contracts § 375 at 220, Illustration 3 to *555Comment a (1979); Annot., 41 A.L.R.2d 905 (1955).
REVERSED.
All Justices concur except UHLEN-HOPP, McGIVERIN, LeGRAND and LARSON, JJ., who dissent.