This month's retirement benefit checks for at least 800 to 900 former federal workers who live in the Rockville area apparently have been lost in the mail, according to the Office of Personnel Management officials, who said persons missing their checks should write OPM for replacements.

Replacement checks should be available about 10 days after the agency is notified, an OPM official said last night.

"We don't know yet what happened or how many people may be affected," said Andrea Minniear, OPM's deputy assistant director for retirement programs. "We're waiting for the post office to get back to us."

The missing checks were mailed Monday from Chicago and addressed to Rockville area residents living in at least three ZIP code districts, 20850, 20851 and 20853, Minniear said. Maryland Rep. Michael Barnes' offices started getting calls from residents in these areas Tuesday about not getting their checks.

An OPM official said last night that retired federal workers or survivors of federal workers who haven't received their checks for January should write the agency's Office of Retirement Programs. Mail should be addressed to Annuitant Services Division, Priority Correspondence Unit, Room 2H-37, 1900 E St. NW, Washington, D.C. 20415. Retirees or the person to whom the check normally is made out must sign the letter, provide an account claim number and a return address.