Single dose treatment with stannic chloride (4 micrograms/ml) on human lymphocytes in vitro revealed a significant increase of chromosomal aberrations. These showed a distinct relationship with the donor's age. Single and isochromatid breaks including gaps, premature chromosome condensation, irregular staining, stretching of the centromere and interchange, i.e. quadriradial exchange of chromosome arms, and also aneuploidy, were observed.