Peripheral blood lymphocytes from 32 patients with malignant melanoma were tested for cell-mediated cytotoxicity (CMC) against cultured autologous melanoma cells. Effector cells were prepared from venous blood by defibrination, gel sedimentation, nylon column filtration, and lysis of remaining erythrocytes with NH4Cl. Melanoma cells prelabelled with [3H])proline were used as target cells in a 40-h assay and CMC was evaluated against standards obtained with blood lymphocytes from the least reactive normal donor. Reproducible autologous CMC was detected in 18 of 32 patients in a series of 367 total tests. CMC correlated with tumor volume (5-500 cm3) but not with tumor stage or DNCB reactivity. Preliminary results indicated that autologous CMC was not affected by treatment with DTIC, dexamethasone, intralesional BCG, radiation therapy, or partial surgical excision. Lack of consistent CMC in 14 patients could not be attributed to a measurable decrease in general immune capacity or to increased resistance of the patients' melanoma cells to CMC in general. Fibroblasts were more resistant to CMC than melanoma cells, and therefore of questionable value for defining specificity in direct tests.