Twenty-one patients with malignant melanoma stage III and IV were treated by intradermal injections of MER--methanol extraction residue of Baccillus Calmette-GuΓ©rin (BCG). Twelve of the patients, all with active disease, received chemotherapy in addition. Tests of skin reactivity to recall antigens and in vitro lymphocyte transformation by phytohemagglutinin (PHA), purified protein derivative (PPD), Canadidine, and Varidase were performed in 19 of the patients before and after the MER therapy. The skin reactivity improved in 8 patients, 5 patients maintained their cutaneous competence, 3 remained incompetent, and in 3 other patients a decrease in reactivity occurred. Improvement or maintenance of cutaneous response was more common in stage III than in stage IV patients. The in vitro lymphocyte stimulation tests have shown that reactivity increased or remained at a high level in 9 of 11 patients in stage III. It dropped or remained low in 2 patients in respect to all antigens used in spite of repeated MER injections. In most patients with stage IV disease no improvement could be detected. MER seems to be a potent immunostimulator in patients in stage III. The side effects were tolerable in all patients and consisted of local reactions and influenza-like symptoms. In 4 patients treated with MER by intralesional injections, transient flattening of nodules occurred but no complete regression was observed.