CB3717 is a quinazoline antifolate whose cytotoxic activity is mediated by inhibition of thymidylate synthase (TS). A phase I clinical trial commenced in September 1981 and 99 patients have received 296 treatments. Doses were dissolved in 0.15 mol/L NaHCO3 (pH 9.0) at a concentration of 4 mg/mL infused over one hour or in a total volume of 1 L infused over 12 hours. Doses were repeated every 3 weeks. The starting dose of 140 mg/m2 was escalated to 600 mg/m2. Renal toxicity, detected by a decrease in the 51Cr EDTA clearance, was dose-related and occurred in seven of ten patients receiving greater than 450 mg/m2. Reversible hepatic toxicity often associated with malaise occurred in 223 of 288 assessable courses (77%). Fifty-nine courses (20%) were associated with increases in alanine transaminase (ALT) levels to greater than 2.5 times the upper limit of the normal laboratory range. Increases in alkaline phosphatase levels also occurred, but were less marked. The severity and prevalence of these elevations were unaffected by the duration of the infusion. A self-limiting rash appeared in 12 patients and a radiation recall reaction was seen in two. Leukopenia developed in 17 patients (WBC less than 3 X 10(9)/L), and thrombocytopenia occurred in six patients (platelets less than 100 X 10(9)/L). The mean leucocyte nadir occurred on day 10 and was followed by recovery at 11 to 19 days. Neither the incidence nor the severity of any of these latter toxicities was dose related. The maximum tolerated dose was in the region of 600 mg/m2 with renal toxicity being dose limiting, although the inter-patient variation did not allow a precise definition. Seventy-six patients were evaluable for response. Responses occurred at doses greater than or equal to 200 mg/m2 and were ovary, one complete response (CR), one partial response (PR), seven minor responses (MR) in 30 cases; breast, two PRs and one MR in eight cases; adenocarcinoma of the lung, one MR in 5 cases; mesothelioma, one PR in five cases; and colon, two MRs in four cases. CB3717 has activity in heavily pretreated patients. The recommended phase II dose for good-risk patients is 400 mg/m2 using the one-hour infusion schedule of administration.