Thirty patients were entered in a phase I trial of aziridinylbenzoquinone (AZQ). These patients were treated with a single iv dose of AZQ daily for 5 days at doses ranging from 0.5 mg/m2 to 10.0 mg/m2. The dose-limiting and only significant toxic effects were leukopenia and thrombocytopenia, which appeared with relative consistency at iv dosages of 4.0--4.5 mg/m2/day x 5 in patients with extensive prior treatment and 7.0 mg/m2/day x 5 in patients with little prior treatment. One patient with colon cancer achieved a partial response of 4+ months. No other evidence of antitumor activity was observed. Pharmacologic studies revealed that the drug disappeared from the plasma in a biphasic manner (alpha-half-life = 1.5 minutes, beta-half-life = 18 minutes). With our method of analysis by high-performance liquid chromatography, no metabolites were found in the serum.