L-alanosine (NSC 15353) is a newly developed antitumor antibiotic which acts as an inhibitor of purine intermediary metabolism. Experimental antitumor activity was demonstrated in a variety of murine neoplasms. A Phase I trial was undertaken on a daily x 5 (d x 5) schedule in 22 evaluable patients. Dose limiting toxicity was an oral mucositis characterized by beefy red oral, lingual and pharyngeal erythema. The maximum tolerated dose is 320 mg/m2/d x 5 every three weeks. The recommended dose for Phase II evaluation is 160 mg/m2/d x 5 every three weeks.