Using semi-solid clonal assay systems, the cytocidal effects of chemotherapeutic drugs on hematopoietic progenitors (CFU-gm, BFU-e, CFU-e) were compared with their effects on K-562 human leukemia cells. The chemotherapeutic drugs used for these experiments were daunorubicin, cytosine arabinoside, vincristine, methotrexate, bis-chlorethyl-nitrosourea and chlorozotocin. After the cells were exposed for either 1 or 24 h to several concentrations of each drug, clear differences in suppressive effects were seen. The experimental applications of this technology and the clinical implication of these results are discussed.