Trimethyltin chloride was found to induce cytotoxic damage in vitro in human peripheral blood lymphocytes. Two concentrations (0.5 μg and 1.0 μg) were added to lymphocytes from male and female subjects in mitogen stimulated and serum supplemented culture medium for 72 h. A dose-related increase of inhibition of replication index (P < 0.01) and cell division (P < 0.001) was observed. The frequencies of abnormal cells and chromosomal aberrations such as chromatid and chromosome breaks, dicentrics, triradial and quadriradial configurations were increased significantly (P < 0.001), as were micronucleus counts (P < 0.001) and sister chromatid exchanges (P < 0.001). Endoreduplication, an extremely rare spontaneous event in human lymphocytes, was observed in a few cases at all dose levels. The effects were more pronounced in lymphocytes obtained from habitual smokers. © 1989, Sage Publications. All rights reserved.