The effect of different cytostatic chemotherapeutic drugs on the frequency of sister chromatid exchanges and growth kinetics of the bone marrow cells in normal adults was studied by means of an in vitro bromodeoxyuridine chromosome labeling method. Cytosine arabinoside, harringtonine, methotrexate, vincristine, and 6-mercaptopurine were tested. Three of these agents induced significantly high frequency of sister chromatid exchange (p less than 0.01 or p less than 0.05). The sister chromatid exchange frequency induced by cytosine arabinoside and that induced by 6-mercaptopurine were higher than those of the control group. The increased frequency of sister chromatid exchange predominated in C and A group chromosomes. At the concentration adopted by the authors, the growth kinetics were little influenced by these drugs.