Phase I study on antimetabolic carcinostatic DMDC was conducted at 16 medical institutions nationwide for patients with various types of malignant tumors. DMDC was administered by intravenous infusion as per the following three schedules: single administration, single repeated administration, and 5-consecutive-day administration. The safety of the compound was examined single administration in 16 patients, by the single repeated administration in 5 patients, and by the 5 consecutive-day administration in 7 patients, for a total of 28 patients. In the single administration trial, 200 mg/m2 (1 n) was given as an initial dose, then increased stepwise to 450 mg/m2 (2.25 n). The single repeated administration trial was conducted at a single dose of 300 mg/m2. One treatment course lasts until recovery from side effects and abnormalities in laboratory test values. As a general rule, the administration was repeated for 2 treatment courses or more. In the 5-consecutive-day administration trial, an initial dose was 30 mg/m2/day (1 n), and increased to 40 mg/m2/day (1.3 n). The dose-limiting factors for both the single and 5-consecutive-day administration trials were decreases in the numbers of leukocytes and neutrophils. The maximum tolerated dose for single administration trial was over 400 mg/m2 (2 n), and for the 5-consecutive-day administration trial 40 mg/m2 (1.3 n). The decrease in the number of leukocytes and neutrophils for both the single administration and 5-consecutive-day administration trial reached its nadir one to two weeks after administration, and recovered in about one week. In the single repeated administration trial, the administration interval for patients who had completed 2 courses was 2 approximately 3 weeks. The plasma half-life of DMDC in the final phase of elimination in the single administration trial was 5.2 approximately 6.3 hours, and no differences were seen among dose levels. The urinary excretion rate was between 32.0 approximately 61.5% until 48 hours after administration. No accumulation was seen in the 5-consecutive-day administration trial. There were no findings to suggest an antitumor effect in the present study. Given the recovery pattern for suppression of marrow, the above mentioned results led us to decide that an recommended method of administration and dosage in an early phase II trial would be 300 mg/m2 per administration by an intravenous infusion every 2 approximately 3 weeks.