OPINION OF THE COURT
Kane, J.At issue in this proceeding is whether the conduct of a grievance representative of petitioner Civil Service Employees Association, Inc. (CSEA), who was found by respondent Public Employment Relations Board (PERB) to be grossly negligent, constitutes a breach of CSEA’s duty of fair representation and is thus a violation of Civil Service Law § 209-a (2).
On September 10, 1981, intervenor, Luis Diaz, an employee of the State Department of Mental Hygiene, was served with a notice of discipline charging him with mistreatment of a patient at Pilgrim Psychiatric Center and seeking the termination of his employment. A grievance representative of CSEA, Vito Bertini, assisted Diaz in filling out a grievance form, wherein Diaz waived his right to an agency level meeting and requested to proceed directly to arbitration. As a result of a series of mistakes and omissions on the part of Bertini and other CSEA representatives, the charges were not timely filed with the American Arbitration Association and, accordingly, the grievance was dismissed. Thereafter, Diaz’s employment was terminated.
An improper practice charge was filed by Diaz with PERB alleging, essentially, that (1) CSEA was grossly negligent in its untimely appeal to arbitration, (2) the untimely appeal was knowing, deliberate and in bad faith, and (3) CSEA’s refusal to seek judicial review following arbitration was in breach of its obligations. All of the charges were dismissed by an Administrative Law Judge after a hearing and were confirmed by PERB, except that PERB found that although CSEA was not grossly negligent in its handling of Diaz’s appeal to arbitration, its failure to adequately train Bertini or provide him with sufficient organizational support amounted to gross negli*432gence. Accordingly, PERB concluded that CSEA breached its duty of fair representation and thereby engaged in an improper practice in violation of Civil Service Law § 209-a (2).
We disagree. In order to establish a claim for breach of the duty of fair representation against a union, there must be a showing that the activity, or lack thereof, which formed the basis of the charges against the union was deliberately invidious, arbitrary or founded in bad faith (see, Smith v Sipe, 109 AD2d 1034, 1035, revd on dissenting opn below 67 NY2d 928; see also, Baker v Board of Educ., 70 NY2d 314).
Accordingly, and in any event, we reject the standard applied by PERB that "irresponsible or grossly negligent” conduct may form the basis for a union’s breach of the duty of fair representation as not within the meaning of improper employee organization practices set forth in Civil Service Law § 209-a. An honest mistake resulting from misunderstanding or lack of familiarity with matters of procedure does not rise to the level of the requisite arbitrary, discriminatory or bad-faith conduct required to establish an improper practice by the union (see, Matter of Trainosky v New York State Dept. of Taxation & Fin., 105 AD2d 525).