SOUTH GREENSBURG, Pa. (AP) _ PPG Industries said it would close its plant here after workers rejected a final contract offer that called for reduced medical benefits.

''A tentative decision has been made that the plant will close, but a date has not been set,'' company spokesman John Ruch said Saturday.

Members of the Aluminum, Brick and Glass Workers International voted 148-59 to reject the proposal. The union walked out about four weeks ago after rejecting the same contract offer.

Workers went on strike at the plant, which makes replacement car windshields, on May 8 when talks foundered over PPG's bid to change the medical plan and lower costs.

''The members have spoken and have evaluated the company's final proposal as being totally unacceptable and concessionary,'' said union negotiator Roy Albert.

Employees at two other western Pennsylvania glass plants and one in Fresno, Calif., walked out after they were presented with similar contract proposals.