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We have investigated the effect of protease inhibitors on hyperthermic cell killing using cultured mammalian cells (L5178Y) and found that protease inhibitors were potent hyperthermia sensitizers. At 37 degrees C, phenylmethylsulfonyl fluoride (PMSF), a serine protease inhibitor, was not cytotoxic at the concentration of 400 micrograms/ml for up to 6 h. When cells were exposed to PMSF (200-400 micrograms/ml) during heating at 43 degrees C, significant potentiation of hyperthermic cell killing was observed. Other protease inhibitors, such as chymostatin and diisopropylfluorophosphate (both are serine protease inhibitors); (2S,3S)-trans-epoxy-succinyl-L-leucylamido-3-methylbutane ethyl ester (cysteine protease inhibitor) and pepstatin-A (aspartate protease inhibitor) showed similar effects. However, when cells were heated at 43 degrees C in the presence of cycloheximide (a protein synthesis inhibitor) together with PMSF, hyperthermic enhancement by PMSF decreased markedly. A decrease in potentiating the effect of PMSF was also noted with thermotolerant cells. These facts suggest that protease inhibitors may exert their hyperthermic cell killing by inhibiting proteases and ubiquitin, which are necessary to degrade denatured proteins induced by heat.