A prospectively controlled randomized clinical trial of adjuvant BCG immunotherapy in patients with stage 1B malignant melanoma (Clark's level 3--5) is described. The combination of intradermal and oral BCG allows approximation of the bacilli to any microscopic foci of residual disease. The trial was activated in May 1975 and to date 107 patients have been admitted to the trial, 49 patients being randomil entered patients, there have been five relapses of 49 patients in the treatment group and ten relapses of 58 patients in the control group. This encouraging trend is not yet statsitically significant at the 5% level (P = 0.17). If evaluable patients with Clark's level 3 and 4 lesions are assessed separately, there is a significant trend in favour of the immunotherapy group (P less than 0.05) with three relapses in the treatment group and ten relapses in the control group.