Eight highly inbred lines, with inbreeding coefficients ranging from 81% to 97% and segregating for eight different B complex alleles were used in this study. Also included were two moderately inbred (40%) heavy-breed lines and one outbred Leghorn line segregating for the blood group alleles, B('1), B('2) and B('19). Birds were blood-typed for B locus alloantigens at 5 weeks of age. Each genetic group was challenged with Rous sarcoma virus subgroups A, B and C. Tests were based on both wing web-challenge (WWC) on 5-week-old chicks and on the chorioallantoic membranes of 12-day embryos (CAM) test;These results indicate that the B complex-linked genes, controlling tumor expression, belong to a multiple allelic series at a single B complex-linked locus. In addition, the results from both the WWC and CAM test support the findings that the tv loci for cellular resistance maps outside the B complex. Also, the statistical evidence verifies the earlier findings that tva and tvc loci are linked. As expected, most of the variation in tumor regression in susceptible birds was linked to the B complex. Thus, part of the genetic variation in tumor regression may be controlled outside the B complex;Genes coding for control of the fate of Rous sarcoma virus subgroup A induced tumors are evidently closely linked to the locus controlling immune response to GAT. The latter maps within the Ir region of the B histocompatibility complex. These findings are important as possible criteria for detecting and selecting disease-resistant fowl.