The clinical trial of KRN 8601 was conducted in patients with neutropenia induced by chemotherapy for lung cancer. Thirty-six patients were treated with KRN 8601 subcutaneously for 14 days once daily at the dose of 50 or 100 micrograms/m2, and the effects were compared with the control phase without KRN 8601 treatment. Both the elevation of neutrophil count and shortened period of neutropenia were observed by the administration of KRN 8601. The efficacy rate was 75% (18/24) at 50 micrograms/m2 and 100% (10/10) at 100 micrograms/m2. A side effect observed was fever in one patients, and in 2 patients, abnormal GOT, GPT and LDH elevation were observed in each. We concluded that KRN 8601 was clinically effective and safe at the dose of 50 micrograms/m2 or 100 micrograms/m2 for neutropenia induced by chemotherapy for lung cancer.