Twenty previously untreated patients with advanced colorectal carcinoma were treated with acivicin given in a 5-day iv schedule at a starting dose of 15 mg/m2, escalating to 26 mg/m2. Therapy was repeated every 3 weeks. Nineteen patients were evaluable for toxicity, which was mild and tolerable overall. Hematological toxicity was minimal, with only one patient developing severe myelosuppression at the starting dose. Nonhematological toxicity was also mild to moderate, occurring in 70% of the patients. Nausea, vomiting, and stomatitis were the most common side effects. CNS toxicity was moderate and occurred in 55% of the patients. Some patients found these symptoms distressing, although the treatment had to be discontinued in only one patient. There were no objective responses among 17 patients evaluable for response. This study indicates that acivicin given in a 5-day schedule is well-tolerated overall.