A lawsuit filed in federal court seeks unpaid overtime for an estimated 3,500 employees of Cintas Corp. of Cincinnati, the nation's largest uniform supplier.
The suit was backed by the garment workers' union UNITE, which is trying to organize the company's 27,000 employees.
Cintas Vice President Karen Carnahan said the suit was "just another tactic by the union in its membership drive."
Union President Bruce Raynor said the company had threatened and fired union supporters, and that a successful suit "will show people that yes, you can stand up to Cintas."
Attorneys who filed the suit in U.S. District Court in San Francisco said drivers in California, Illinois, Michigan and New Jersey were classified improperly as managers and forced to work up to 70 hours a week without overtime pay.
A wave of similar suits have been filed during the last five years. Almost all were settled before trial.