The effect of KW-2228, a derivative of recombinant human granulocyte colony stimulating factor, on neutropenia associated with chemotherapy was studied in pediatric patients with malignant tumor. To patients repeatedly treated with the same chemotherapy, KW-2228 was administered subcutaneously at 1 microgram/kg or intravenously at 2 micrograms/kg once a day in the 2nd course of the chemotherapy. During the administration term of KW-2228, the nadir of neutrophils went up, and the duration of neutrophil count under 500/microliters as well as the interval for recovery of the count to 500/microliters were remarkably shortened, compared with those during the observation term. Further, the duration of fever in patients and the administration days of antibacterial agent were also reduced. The effective rate judged by attending doctors was as high as 78.9%, consisting of 83.3% in 42 cases of subcutaneous administration and 73.5% in 34 cases of intravenous administration. Side effects such as slight skeletal pain and slight fever in one case each (2.3% in total) were improved without treatment. It is concluded that KW-2228 may be a useful drug for treatment of neutropenia associated with chemotherapy in pediatric patients with malignant tumor.