A lawsuit filed in New York says Major League Baseball staffed a profitable "2013 All-Star FanFest" last month almost entirely with more than 2,000 unpaid volunteers.
The federal lawsuit was filed Wednesday in Manhattan.
It seeks unpaid wages and an order to stop the league from soliciting and accepting work from unpaid volunteers.
The lawsuit said recruiting volunteers violates federal and state minimum wage laws.
Lawyers brought the lawsuit on behalf of a Queens man who volunteered for five days at the event. They said they will seek class-action status.
They noted that the league has already invited this year's volunteers to work for free at the 2014 All-Star Game in Minneapolis.
A baseball spokesman did not immediately return a message for comment.