Summary Cells from crossbred embryos from the inbred Reaseheath R and W lines, which were segregating at the tva and tvb loci, and at a possible third locus, tvc, were examined for responses to avian sarcoma viruses of subgroups A, B and C. Responses to subgroup A were independent of those to subgroup B, as were those for subgroups B and C. Genetic analysis of the results suggested absence of linkage between the tvb locus and the tva or tvc loci. Responses to subgroup A and C viruses were strongly associated and were consistent with linkage between the tva and tvc loci, or with the presence of a single locus which controlled responses to both subgroups.