A phase I study of VP was undertaken using the methods of a single (40 cases; range of dose levels 30-540 mg/m2) and 5-day (41 cases; range of dose levels 40-140 mg/m2/day) intravenous administration. The dose-limiting toxicity of VP was moderate to severe leukopenia. MTD was estimated to be 540 mg/m2 for a single and 140 mg/m2/day for 5-day administration. The median days to WBC nadir from the start of therapy and to recovery from reaching the nadir were 10 and 10.5 for single, and 15 and 7 for 5-day administration, respectively. Thrombocytopenia was less frequent and less pronounced than leukopenia. Mild gastrointestinal disturbances and alopecia were frequently observed. Transient hepatic dysfunction, fever, headache, fatigue, dyspnea, hypotension, and pain along the vein were also encountered in a small number of patients. There were no cases with renal, neurologic or cardiac toxicity. Objective tumor regression was seen in one case each of IBL(CR), bladder cancer, non-Hodgkin's lymphoma and ATL (PR). The post-infusion plasma decay of VP in 4 cases given 80-120 mg/m2 by a single administration was biphasic with t1/2 alpha ranging from 0.13 to 0.39 h and t1/2 beta ranging from 3.33 to 4.85 h. No accumulation of VP was found in plasma after five repeated daily doses. Doses of 360-480 mg/m2 by single and 80-100 mg/m2/day by 5-day administration repeated every 3 to 4 weeks can therefore be recommended for phase II studies in good-risk patients.