YK-176 is a newly isolated 2'-deoxycoformycin (DCF), a potent inhibitor of adenosine deaminase, produced by Aspergillus nidulans. In a cooperative phase I study, YK-176 was administered to 22 patients, comprising 18 with adult T-cell leukemia-lymphoma (ATL), two with cutaneous T-cell lymphoma (CTCL), one with lymphoblastic lymphoma of T-cell type and one with carcinoma of the uterine cervix. Doses of YK-176 ranged from 3.0 to 9.0 mg/m2 and were given intravenously for three consecutive days. General malaise, anorexia, nausea, vomiting and low grade fever were frequently encountered, but were transient and not dose-related. At all dose levels hematological toxicities were mild. Two of seven patients receiving 7.0 mg/m2 for three consecutive days developed hepatocellular enzyme elevations (grade 2) and one patient, proteinuria (grade 2). One of two patients given 9.0 mg/m2 for three consecutive days manifested a life-threatening (grade 4) disturbance of consciousness and dyspnea, presumably ascribable to the drug-related toxicity of YK-176. The results suggest that 7.0 mg/m2 i.v. for three consecutive days is the maximum acceptable dose of YK-176. Central nervous system, pulmonary and possibly renal toxicities appeared to be dose-limiting. Out of the 20 patients evaluable for therapeutic response, partial remissions were observed in four, three with ATL and one with CTCL, who received less than 7.0 mg/m2 for three consecutive days. We conclude that YK-176 is an active agent against ATL at doses that may not be associated with prohibitive toxicity. A starting dose of 5.0 mg/m2 for three consecutive days is recommended for further phase II studies on ATL.