Despite current surgical therapy, about 80 per cent of patients with malignant melanoma metastatic to lymph nodes succumb to systemic metastatic disease. To determine if postoperative adjuvant immunization with BCG was an effective systemic treatment in these patients with microscopic subclinical metastatic disease, the clinical course of 42 patients treated by operation alone was compared with that of 84 treated by operation and BCG. At two years, the incidence of metastasis in BCG-treated patients was half that of the control group. BCG was more effective in patients with a smaller tumor burden at the time of initial surgical treatment. In patients receiving BCG adjuvant therapy, 90 per cent with microscopic disease in one lymph node appeared free of disease as compared to 40 per cent with macroscopic disease in multiple nodes. In patients with recurrences, an immunotherapeutic effect was demonstrated by a delay of six months in the time to recurrence. Thus, BCG immunotherapy appears to have an inhibiting effect on the "micrometastases" of malignant melanoma.