Leukocyte migration inhibition (LMI) assays were performed using 3 M KCI extracts to detect cell-mediated immune reactions against tumor-associated antigens (TAA) of swine melanoma. Positive LMI reactivity was 70% or greater in melanoma-bearing swine compared to 10% reactivity or less in normal swine tested with extracts of either fresh or tissue-cultured swine melanoma cells. Positive LMI reactivity was 10% or less in both melanoma and normal swine tested with extracts of normal fetal and adult swine tissues. Extracts of tissue-cultured swine melanoma and human melanoma cells were equally capable of stimulating positive LMI reactivity in melanoma swine and human melanoma patients; normal swine and human donors demonstrated 10% or less positive LMI reactivity to these extracts. Positive LMI reactivity was not stimulated in swine melanoma or human melanoma donors by extracts of tissue-cultured human breast tumor cells or fresh and tissue-cultured mouse B16 melanoma cells. Evaluation of these results indicate that melanoma-bearing swine develop cell-mediated immune reactivity toward TAA on swine melanomas and, inasmuch as LMI assays were performed with allogeneic extracts, suggests the presence of common TAA on swine melanomas. Furthermore, these data suggest that swine and human melanoma cells may share some common TAA and that this common TAA is not expressed on mouse B16 melanoma cells.