As a co-operative study involving 18 medical research institutions, we have made a randomized, controlled study (phase III study) of Bestatin in the treatment of malignant melanoma (stage Ib, II) over the last 4 years. The results were as follows; Bestatin significantly prolonged disease-free intervals by 9-20 month. Bestatin significantly prolonged survival periods by 10-16 months. Side effects of Bestatin, chiefly comprising of reversible gastro-intestinal disturbances, were observed in 7.1% of Bestatin-treated cases. These findings suggest that Bestatin is a very useful agent for treating for stage Ib and II malignant melanomas.