In phase I studies of KRN 8601, single administration and six-day consecutive administration studies were conducted in healthy male adults using intravenous drip infusion and subcutaneous administration. Safety and tolerance to KRN 8601 were confirmed and a dose-related increase of neutrophil counts by KRN 8601 was observed. In an early phase II study, the safety and tolerance to KRN 8601 in cases of neutropenia following cancer chemotherapy were shown at doses of 25-800 micrograms/m2 and an improvement of neutropenia was seen at doses of more than 50 micrograms/m2. In a phase II study, the optimal dose was investigated for subcutaneous administration and intravenous drip infusion in cases of neutropenia induced by chemotherapy for malignant lymphomas. The optimal dose was 75 micrograms/body (about 50 micrograms/m2) for subcutaneous administration and 100-200 micrograms/m2 for intravenous drip infusion. In a phase III study, a double-blind prospective randomized trial comparing KRN 8601 with an inactive placebo against malignant lymphomas was performed and the inhibitory, improvement and recovery-promoting effects of KRN 8601 (75 micrograms/body, sc) on neutropenia were demonstrated.