SummaryThe K region of the H‐2 major histocompatibility complex (MHC) of mice ofH‐2Kkhaplotype codes for two distinct alloantigens. One of these alloantigens, designated k‐common, is expressed by C3HfB/HeN mice (C3Hf). The other alloantigen, designated k‐unique, is not expressed by C3Hf mice. TheH‐2haplotype of C3Hf mice has been classified as kv1 and the variant antigen distinguishing this strain from mice ofH‐2Kkhaplotype has been designated kv1‐unique. Several transplacentally‐induced lung tumours of C3Hf mice express the k‐unique, rather than the expected kv1‐unique, antigen. The immunogenicity of the k‐unique antigen on C3Hf‐derived lung tumours varies with different tumours. In particular, the capacity of the k‐unique antigen to induce radioresistant immunity in C3Hf mice appears to be lost on long term cultured tumour cells even though the tumour remains susceptible toin vivoimmune responses directed against the k‐unique antigen. Alterations in expression and in immunogenicity of unique H‐2‐coded antigens may dictate the nature and efficacy of immune survei