The mutagenicity of a trihalomethane-bromoform (CHBr3)-was assessed by the in vitro sister-chromatid exchange (SCE) assay using human peripheral blood lymphocytes. CHBr3 was found to induce SCEs significantly in a dose-dependent manner. When the cells were exposed to 600 ng CHBr3/ml of the medium, the SCE/cell mean reached a value as high as 18.78 +/- 0.17. Beyond this concentration. CHBr3 proved to be cytotoxic. A protease inhibitor (PI), purified appreciably by affinity chromatography from fieldbean (FB), was able to suppress significantly in a dose-dependent way the high SCE frequencies induced by this specific concentration of CHBr3 (600 ng/ml). Addition of 600 micrograms of FBPI/ml of the medium brought down the CHBr3-induced high SCEs to near (8.80 +/- 0.15) base line or control value (8.45 +/- 0.21). A study of the effect of FBPI on the normal low SCE frequencies in these cells indicated that the FBPI has the intrinsic property to suppress in a dose-dependent manner these SCEs in the lymphocytes. This functional property of FBPI, which is related to its protease inhibitory activity and which is destroyed when it is inactivated by autoclaving, makes it an effective antimutagenic/chemopreventive agent.