The clastogenicity of trimethyltin chloride was evaluated in human peripheral blood lymphocytes with micronucleus counts (MNC) as the endpoint. Two concentrations (0.5μg and 1.0 μg) of trimethyltin chloride were added to lymphocytes of healthy male and female subjects of different age groups, in mitogen-stimulated and serum-supplemented culture medium (RPMI 1640, Gibco) for 48 h at 37°C. A significant increase in micronucleus counts was observed with both doses, which was more pronounced with the lower dose. ANOVA in male and female donors revealed significant differences between age groups (P<0.001), chemical concentrations (P<0.001) and for the interaction of these 2 factors (P<O.05 in females only). However, no regular increase or decrease in MNC frequencies was observed with the donor's age. Higher frequencies of MNC enhancement were observed in male individuals than in females. © 1990.