Clastogenic effects of antimony trichloride, used in small industries, were monitored in laboratory bred white Swiss mice in vivo, following oral administration by gavaging, after 6, 12, 18 and 24 h. Chromosomal aberrations were principally breaks and damaged cells observed from bone marrow preparations. The frequencies of chromosomal aberrations were directly related to the dose used and were significantly higher than the control. The chemical did not alter the frequency of dividing cells to any significant level.