Sixty-one patients with advanced disseminated cancer were given progressively increasing doses of pyrazofurin to evaluate toxicity patterns and to establish the dosage that produces maximum therapeutic effect with clinically tolerable toxicity. The drug was given by intravenous injection over 5-day courses repeated every 2--3 weeks. Toxic reactions included stomatitis, myelosuppression, skin rash, erythema, proctitis, and occasional nausea and vomiting. Stomatitis was the dose-limiting toxicity and it occurred in 32 patients. Myelosuppression was mild to moderate. Of 75 evaluable courses for marrow toxicity, leukopenia occurred in 14 and thrombocytopenia in 28. Thrombocytopenia was apparently dose-independet. Marrow recovery was complete by day 21 of therapy. Twelve patients developed mild or severe cutaneous toxicity depending on dose. When mild, the skin changes consisted of self-limited erythema or rash, and when severe, bullous lesions and skin ulcers were also observed. Proctitis occurred in six patients and was associated with severe stomatitis. Nausea and vomiting were occasional and mild. There was no evidence of liver or renal toxicity. All toxic manifestations other than marrow toxicity were dose-related. No responses were observed. A reasonable dose schedule is 45 mg/m2/day X 5 repeated every 3 weeks. We recommend that Phase II studies be pursued particularly in diseases that have been shown to be sensitive to the drug.