Box office personnel walked off their jobs at the Mechanic Theatre yesterday, hours before the opening of the season's first show, 42nd Street.

The breakdown with Local 868 of the union that represents ticket sellers for the Mechanic concerns the status of the box office manager, said David M. Anderson, president of the theater management group of Clear Channel Entertainment. Clear Channel believes the managers should be exempt from the union, Anderson said.

However, Walter Cahill, an International Alliance of Theatrical State Employees official, played down that issue, saying, "It's an economic strike. It's largely over wages."

Last night's performance was expected to go on as scheduled.