This study presents results on 2'-2'-difluoro-2'deoxycytidine's (dFdC: gemcitabine) in vitro toxicity on peripheral blood CFU-GM and BFU-E obtained from healthy volunteers. Peripheral blood mononucleated non-adherent cells were cultured according to standard methods with continuous exposure (13 days) to dFdC (4,40 and 400 pmol/L) or following 1 hour's, incubation with increasing drug concentrations (1, 10, 100 mumol/L). The results indicate that dFdC has a marked dose-dependent inhibitory effect on the in vitro growth of peripheral blood hemopoietic progenitors., No significant differences were observed for the growth inhibition induced on CFU-GM and BFU-E. Continuous exposure to dFdC gave an IC50 of 4 pmol/L for both CFU-GM and BFU-E. In four chemotherapy naive patients affected by tumors of different type treated with three standard courses of dFdC the variations in the peripheral blood of hemopoietic progenitor level were determined. Patterns of changes were different, but a marked and sustained decrease of both CFU-GM and BFU-E was observed in one case only. The contrast between the apparently rather mild clinical hemotoxicity of dFdC and its in vitro dramatically potent inhibitory activity on hemopoietic progenitors is discussed.