In vitro grown B16, an H-2b melanoma cell line, was found to kill B10 (H-2b), B10.BR (H-2k), CBA (H-2k), and BALB/c (H-2d) but not B10.D2 (H-2d) mice. BALB/c mice could be protected against a lethal dose of B16 by co-administration of H-2b spleen or peritoneal exudate cells, suggesting B16 was susceptible to an immune response but unable to induce it. However, evidence for the immunogenicity of B16 tumour in vivo was shown in two ways--(i) by immunization with irradiated B16 before lethal tumour challenge, and (ii) by demonstration of specific immunity in BALB/c mice which survived a low dose of viable B16 tumour. The resistance of most B10.D2 mice to doses of B16 tumour which were lethal to four other strains of mice was also compatible with the immunogenicity of B16 in vivo. By contrast, B16 was unable to induce a primary Tc-cell response in vitro but was able to induce a secondary response. The significance of these results for theories of T cell immunity to foreign tissue and for improving immunogenicity of tumours is discussed.