The purified trypsin inhibitor (TI) isolated from nematode Ascaris suum was tested in vitro for chromosome aberrations and sister chromatid exchanges (SCE). TI was obtained from the musculocutaneous sac homogenate of adult Ascaris by the modified method of Rola and Pudles. The inhibitor was isolated and purified from the SF5 fraction of proteins by gel filtration on Sephadex G-50 and electrophoresis SDS-PAGE of the obtained fraction after molecular filtration. TI showed a high inhibitory activity against crystalline trypsin (18.8 Kassell's units/mg of protein). Genotoxicity assessment of TI was carried out on metaphase plates received from peripheral blood lymphocyte macroculture (48 h-test of structural chromosome aberrations and 72 h-test of SCE), without exogenous metabolic activation. TI was tested in doses: 25, 50 and 100 microg per mL of culture. Kinetics of cell divisions was determined by the replication index (RI). We found that TI in vitro did not induce chromosome aberrations. It induced a higher number of SCE per cell but less than double frequency as compared to the control. The difference was significant only for the dose 50 microg/mL. For all doses, replication index (RI) values were significantly higher and mitotic index (MI) values were significantly lower than in the control. Thus the Ascaris trypsin inhibitor did not show any genotoxic properties but exhibited a mitostatic activity.