Twenty-two patients were given progressively increasing doses of Cytembena to determine toxicity patterns and to establish a dosage which produces definite but clinically tolerable toxicity when the drug is given by intravenous injections in a 5-day intensive course. Toxicity consisted primarily of nausea, vomiting, arm pain, and transiently decreased renal function. At higher doses, an "autonomic-storm" phenomenon was observed consisting of hypertension, tachycardia, tachypnea, hyperperistalsis, frequent explosive defecation, facial flushing and paresthesias, and chest pain with accompanying ischemic EKG changes. There was no evidence of mucocutaneous, hepatic, or hematologic toxic effects. Toxicity was dose-related, first being recognized at a daily dose of 300 mg/m2 and becoming clinically intolerable at a daily dose of 475 mg/m2. No permanent damage was observed in any of the organ systems monitored. An acceptable treatment regimen for most patients is 400 mg/m2/day for 5 days. Patient discomfort can be reduced by dividing each day's dose into two intravenous injections given at an interval of at least 6 hours. Coronary artery disease and impaired renal function should be contraindications to Cytembena therapy, and caution should be employed in the patients with significant impairment of liver function. Two of 22 patients, both with far-advanced carcinoma and previous chemotherapy failures, showed a favorable objective response to Cytembena therapy. Phase II studies to assess the magnitude of the drug's antineoplastic activity seem warranted.