Twenty patients with either stage II or III metastatic malignant melanoma were treated with vaccinia virus injections into the tumor nodules. Average survival was 32.2 months for stage II and 4.6 months for stage III. Delayed hypersensitivity skin tests and 2,4-dinitrochlorobenzene sensitization were positive. Indirect immunofluorescence demonstrated humoral antibodies. Cellular cytotoxicity, as demonstrated by the colony inhibition technique, was strongly positive in all patients who made a good response to immunotherapy. Blocking antibodies were not found in the treated group. Vaccinia virus immunotherapy may act by activation of specific immune mechanisms or may reflect a nonspecific, cytotoxic, inflammatory reaction. This study supports the concept that vaccinia virus may activate the production of cell-mediated, cytotoxic immunity against melanoma cells. This cytotoxic immunity has been isolated with transfer factor and given to other patients with metastatic melanoma.