Comparison of the clastogenic effects of antimony and bismuth used as trioxides, when administered orally by gavaging to laboratory bred male mice, showed that the former was more strongly clastogenic than the latter. Three doses of each chemical (400, 666.67, and 1000 mg/kg body wt), corresponding to 1/50, 1/30, and 1/20 of oral LD50 of antimony trioxide, were fed daily to sets of mice up to 21 d. Animals were sacrificed on day 7, 14, and 21 of the experiment. Chromosomal aberrations and mitotic index were studied from bone marrow cells following a colchicine-air drying Giemsa schedule. The frequencies of chromosomal aberrations induced by both chemicals were directly proportional to the dose used and the duration of exposure, indicating their cumulative effects on the organism. The highest dose of antimony, given for the longest period was, however, lethal. Effects on germ cells, as shown by screening for sperm head abnormalities, were not significant. © 1993 The Humana Press, Inc.