A total of 48 patients with various malignant disorders, mostly from hemopoietic organs, entered to phase I study of MCNU given orally. Fifty-six percent of the patients given MCNU at the doses of 50-125 mg/m2 complained of gastrointestinal symptoms including nausea and vomiting, which were however mild and well tolerated. In the following study employing administration of 50 mg/day of MCNU for consecutive 2-6 days, the gastrointestinal toxicities were reduced to 26.1%, and hematological toxicities of delayed leukopenia and thrombocytopenia were derived 4-6 weeks after oral intake of the drug. The hematological recovery required 1-2 weeks after the nadir of leukocyte and thrombocyte count. A recommended dose for phase II study of MCNU by the route of oral administration was 50 mg/body/day for consecutive 4-6 days in every 8 weeks interval. The peak value of blood concentration of MCNU was obtained 60-120 minutes after p.o. administration at the doses of 50-100 mg. Elimination half-life was estimated to be 40-45 minutes. No concentration of the drug was detected in blood 24 hours after the administration.