Two B complex genotypes, B1B1 and B19B19, of outbred line S1, were tested for low and high immune response to GAT, from which four recombinants were recovered: B1B1 GAT-hi and -lo, and B19B19 GAT-hi and -lo. Also included in the study were birds of B2B2 genotype with an intermediate level of immune response to GAT. A total of 225 birds of these groups were challenged with the Bryan strain of Rous Sarcoma virus subgroup C, RSV (RAV-7), by inoculation into the wing web at five weeks of age. The B1B1 genotype had the lowest percentage of regressors (17.6%), B19B19 had the highest (42.2%), and the B2B2 genotype was intermediate (23.7%). Combining the results of GAT response over the B1B1 and B19B19 genotypes, 14.0% of GAT-lo and 37.8% of GAT-hi regressed their tumours, respectively. The highly significant (P less than or equal to 0.01) difference between the combined GAT-hi and -lo groups would suggest that the Rs locus controlling tumour regression induced by the subgroup C virus is closely linked to the region controlling immune response to GAT, but the data also provides evidence that the B-F region of the B complex also plays an important role in RSV-induced tumour regression.