We evaluated the potential of the B16 melanoma of mice as a model system for BCG immunotherapy of malignant melanoma. We studied a variety of treatment protocols: a) BCG given simultaneously but separately with a small number of B16 cells significantly inhibited tumor growth in only three of eight experiments. b) BCG injected directly into the tumor stimulated tumor growth in three of three experiments; the stimulation was at least partially attributable to the nutrient medium in which the BCG was suspended. c) The B16 tumor was weakly immunogenic and the addition of BCG to a tumor cell vaccine offered little improvement in subsequent resistance to tumor cell challenge: d) In a model of postsurgical residual tumor, metastatic to regional lymph nodes, BCG and tumor cell vaccination did not alter the development of nodal metastases. The B16 melanoma was not a useful model system for BCG immunotherapy, because the tumor inhibition was feeble, inconsistent, and not associated with augmented tumor immunity.