In this study we compared how different concentrations and periods of incubation of anthracyclines, amsacrine, and cytosine arabinoside would affect normal hematopoietic bone marrow cells in terms of interindividual differences in toxicity, the age of the donor, and the proliferative capacity of the bone marrow. Bone marrow was obtained from 36 donors in connection with bone marrow transplantation. After separation the mononuclear cell fraction was incubated with doxorubicin, 4-epidoxorubicin, daunorubicin, idarubicin, aclarubicin, mitroxantrone, amsacrine, and cytosine arabinoside for 1 h, for 3 h, or continuously. The cells were thereafter cultured in soft agar and CFU-GM were counted after 10-12 days. The results showed a large interindividual variation in toxicity for all drugs tested. Daunorubicin, idarubicin, aclarubicin, and mitoxantrone had a pronounced cytotoxic effect after 1 h of incubation. Doxorubicin and 4-epi-doxorubicin showed the greatest cytotoxic effect after 3 h and were also more toxic to normal bone marrow cells from donors over 40 years of age. Ara-C had a low cytotoxic effect after 1 and 3 h of incubation, even at high concentrations, but exerted a pronounced degree of toxicity during continuous incubation. Daunorubicin, idarubicin, and ara-C also showed increased toxicity to cell samples with a low proliferating capacity in the control. The conclusions drawn from these results are that interindividual variation, proliferation capacity, incubation conditions, and the age of the donors are factors of importance in the toxicity of drugs to normal hematopoietic bone marrow cells.