Recent studies suggest that the gene locus controlling the fate of tumors induced by Rous sarcoma virus (RSV) is linked to the B histocompatibility complex. Birds carrying the dominant allele regress the tumor; homozygous recessives being unable to do so, develop large tumors and die. These are called progressors. The Bryan strain of RSV was inoculated into 220 6 week old Leghorns homozygous for B1B1, B2B2, or B19B19 of which the percentages of progressors were 79, 22 and 56, respectively. The balance of each were regressors and survived. The B1B1 test birds were derived from special matings, i.e., high and low immune responders to the amino acid polymer, GAT. Of 67 tests progeny of the B1B1 GAT-low mating, 63 or 94% proved to be progressors, and 6% were regressors. Of 84 test progeny of the B1B1 GAT-high matings, 67% were progressors, and 33% were regressors. The difference between the high and low GAT responders is highly significant and indicates that the locus controlling the fate of RSV-induced tumors is closely linked to the locus controlling immune response to GAT. The latter maps within the Ir region of the B histocompatibility complex. © 1979 Springer-Verlag New York Inc.