Don Fehr, executive director of the Major League Baseball Players Association since 1985, will relinquish the position on Monday, according to a report on ESPN.
He will be replaced by general counsel Michael Weiner, pending board approval, a source told ESPN.
Fehr, 60, succeeded Marvin Miller as union chief when he was named acting director in 1983, and he was elected executive director in December 1985.
Fehr joined the Players Association as general counsel in August 1977. Two years prior, while with the Kansas City law firm of Jolley, Moran, Walsh, Hager & Gordon on behalf of the union, he worked on the landmark case involving pitchers Andy Messersmith and Dave McNally, which opened the door to free agency.