Thirty patients with various solid tumors were treated with 5-azacytidine. Total doses ranged from 1.0 to 24.0 mg/kg and were given over a minimal period of 8 days. The major toxic effect was hematologic with significant leukopenia and thrombocytopenia usually occurring 20-30 days after the start of therapy, especially at higher dose levels. The marrow depression lasted 1-5 weeks and was fully reversible. Nausea and mild diarrhea were common following injection of the drug. Serum glutamic oxaloacetic transaminase levels rose in several patients. No other evidence of hepatic toxicity was seen. Objective remissions were noted in seven of 11 patients with cancer of the breast, two of five with melanoma, and two of six with cancer of the colon.