Various facts are now known about the relative lymphoma resistance of a group of tetraparental AKR reversible CBA/H-T6 chimaeras derived by early embryo aggregation. Firstly, their tumour resistance is not due to the lack of the lymphomaprone AKR cells. Secondly, results showing titres of MuLV-gs antigen comparable with, and occasionally in excess of, those in the AKR suggest that the tumour resistance of the chimaeras is unlikely to be due to a lack of oncogenic leukaemia virus. However, in marked contrast to the AKR, antibody-viral antigen renal complexes in the chimaeras were minimal. Lack of viral antigens could not explain the relative lack of renal complexes. Absence of the corresponding anti-viral antibody is the most likely explanation and this has to be attributed to the CBA component of the tetraparental AKR reversible CBA/H-T6 chimaeras. We suggest that with tolerance to the leukaemia virus being maintained and in the absence of anti-viral antigenic complexes, tumour-specific sites can be recognized and thus tumours are eliminated. This hypothesis remains to be proven.