The ability of CSF-HU (P-100) to inhibit and improve the leukocytopenia and granulocytopenia which occur following cancer chemotherapy was investigated in a double-blind study which included an inactive placebo. The drug was administered, 2 vials/day (8 X 10(6)U of P-100), by intravenous drip infusion consecutively for 7 days starting from the 5th day after cancer chemotherapy was initiated. The total cases included in the study numbered 261. The efficacy rate was 53.0% for the P-100 group and 38.9% for the placebo group, while the usefulness rate was 54.8% for the former and 38.1% for the latter. In either case, statistically significant differences were observed in favor of the P-100 group. The reduction in the number of days before the leukocyte count returned to 4,000/mm3 was statistically significant in the P-100 group. While side effects appeared in both the P-100 (4.8%) and the placebo (4.4%) groups, none of them were considered to be serious. Judging from these results, p-100 was considered to be a useful therapeutic drug for the treatment of leukocytopenia and granulocytopenia following cancer chemotherapy.