The in vitro effect of ciprofloxacin and pefloxacin on growth of normal hematopoietic progenitor cells and on leukemic cell lines was investigated. Ciprofloxacin and pefloxacin caused dose-dependent inhibition of colony formation both from normal bone marrow cells and from the leukemic line K-562 cells. This inhibition exerted by ciprofloxacin and pefloxacin was statistically significant at concentrations of 25 micrograms/ml and above. Ciprofloxacin appeared to be the most potent inhibitor of colony formation among the antimicrobial agents tested. Although the inhibitory effect of pefloxacin on normal hematopoietic stem cells was similar to that of cefazolin and chloramphenicol, the inhibitory effect of pefloxacin on leukemic cells was more prominent than that of cefazolin and chloramphenicol. In a proliferation assay in liquid culture of the cell line HL-60, ciprofloxacin and pefloxacin caused a dose-dependent inhibition of cell proliferation. Both drugs failed to induce cellular differentiation, as assessed by the nitrogen blue tetrazolium dye reduction assay. In therapeutic concentrations no cumulative toxic effect of the combination of ciprofloxacin with cytosine-arabinoside, vincristine, actinomycin D and doxorubicin on colony formation by HL-60 cells was observed. It is concluded that ciprofloxacin does not exert in vitro inhibitory effect on human leukemic cells when assayed at concentrations of less than or equal to 5 micrograms/ml. However, at concentrations of 25 and 50 micrograms/ml of ciprofloxacin alone and in combination with several antineoplastic agents exerts an inhibitory effect on colony formation.