The clinical phase I study of TNP-351, an antifolate drug having a novel structure, was performed through a multicenter cooperative program in 40 patients with solid tumors. The test drug was used on dosage schedules of single and daily doses for 5 or 3 days (by intravenous drip over 30 minutes, respectively). From the daily administration for 5 days, severe adverse reactions such as myelosuppression, became manifest at 5 mg/m2 (1n). This schedule was then switched to daily administration for 3 days. Administration of the test drug was initiated at a dose of 5 mg/m2. On a single-dose schedule, the dose was increased up to 100 mg/m2 (20 n), and on the 3-day daily administration schedule, up to 10.8 mg/m2 (2.2 n). Consequently, 26 of the study patients received single doses; three of them the 5-day daily administration, and 11 the 3-day daily administration. The dose-limiting factors were leukopenia and thrombopenia on both the single-dose and 3-day daily administration schedules. MTD was 100 mg/m2, and MAD, 75 mg/m2 for the single-dose schedule; and 10.8 mg/m2 and 9 mg/m2 for the 3-day daily administration schedule. WBC and platelet counts fell to nadirs at 1-2 weeks on either the single-dose or 3-day daily administration schedule, and it took the respective parameters about 1 week to recover. Subjective and objective adverse reactions to the test drug consisted of digestive tract disorders manifested as stomatitis, anorexia, nausea and vomiting; and laboratory abnormalities such as elevations of GOT and GPT in addition to the myelosuppression. Many of these adverse reactions subsided within 3 weeks after initiation of TNP-351 treatment. On the single-dose schedule, the test drug occurred chiefly in unchanged form in the blood, and in this form it disappeared from the blood biphasically with an alpha phase of 0.29-0.95 hours, and a beta phase of 7.8-14.4 hours. This disappearance pattern did not vary with an increase in dose. The 24-hour urinary excretion rate of the unchanged form amounted to 42-62% of the administered doses. On the 3-day daily administration schedule, the test drug was not accumulated in vivo. In the present study, two patients with malignant fibrous histiocytoma responded to the test drug with tumor regression. The results suggested that the recommended dosage regimen for the clinical early phase II study of the test drug should comprise a course of 9 mg/m2/day (by intravenous drip infusion over 30 minutes) every day for 3 days, which should be repeated every 3 weeks.