Vaccinia melanoma oncolysates (VMO) were tested in a Southeastern Cancer Study Group (SECSG) Phase I/II trial. Forty-eight patients with high-risk Stage I or pathologic Stage II disease were placed on study at six different dose levels and two different treatment regimens. Patients were monitored for toxicity to the VMO after each injection. Patients' sera were tested for anti-human melanoma reactivity with the Staphylococcus Protein A (SpA) assay. Toxicity was minimal at all doses tested. In only 2 of 19 patients on delayed treatment did reactivity develop in the SpA assay by 6 months. However, 13 of 23 patients on immediate treatment showed reactivity, including 8 of 8 at the two highest doses. Since the VMO appears to be safe at all of the doses tested, and because of the immunogenicity of the VMO at the higher doses as demonstrated by the SpA assay, the 2-mg dose level, for immediate treatment, was chosen for use in future trials.