The effects of milk heat treatment and cooking conditions during cheese manufacture on plasmin activity and proteolysis during ripening were determined. Milk heat treatments studied were raw( no heating), 63degreesC for 30 min and 75degreesC for 1, 5 or 10 min. In separate experiments, cooking temperatures of 38, 43, 48 and 55degreesC were examined. Varying cooking temperature did not significantly affect cheese composition except that moisture content was significantly lower (P < 0.05) at higher cooking temperatures and salt content also significantly (P < 0.001) differed between treatments. Increasing cooking temperature increased plasmin activity and plasminogen activation during ripening, but decreased chymosin activity, as seen from urea-PAGE analysis. Increasing severity of milk heat treatment had no significant effects on cheese composition, but decreased plasmin activity in cheese and, overall, had less significant effects on proteolysis during ripening than cooking conditions. Thus, the importance of activators and inhibitors of the plasmin system, and their heat stability, in cheese ripening, and the effect of processing, conditions thereon, were apparent.