In vitro colonogenic assay of mouse marrow granulocytic progenitor cells (spleen and agar colony forming cells) was carried out to determine the cytotoxic effect of 1-B-D-arabinofuranosyl cytosine on granulopoiesis. Mainly, the drug cytotoxicity on progenitor cells depended upon exposure time. Endotoxin stimulated colony forming cells showed enhanced cytotoxicity with shorter (25 min) exposure time as compared to a longer exposure of 60 min. A 60 min exposure was found to give maximum cytotoxic effect on spleen colony forming cells. With prolonged incubation, the cytotoxic effect of the drug on spleen colony forming and agar colony forming cells was more or less the same. Incubation of granulocytic precursor cells with the cytotoxic drug, in vitro, produced effects similar to those previously reported with in vivo drug administration. Therefore, some of these techniques can be employed as improved screening models for evaluating the cytotoxic agents in clinical trials.