37 patients with cutaneous malignant melanoma (MM) were randomized, after surgical removal of tumor, to either no further treatment or to adjuvant treatment by the thymic factor thymostimulin (TS; Tp-1 Serono). 26 patients were with primary (stage I) MM (tumor thickness greater than 1.25 mm), and 11 patients with local or regional disease spread (stage II MM). Randomization was done separately for each disease stage. Life table analysis of the results indicates that significantly (p less than 0.01) more patients were free of disease at one year on study if treated by TS than if left untreated. Probability of survival was also better (p = 0.05) in the TS-treated group at 15 months on study. No side effects, toxic or allergic, were observed under TS treatment.