Gs is an allele of chickens for the expression of endogenous avian leukosis virus-related core (gs) and envelope (chf) antigens. Progeny of a genetic cross in which Gs was segregating were analyzed for endogenous viral DNA as well as for the expression of endogenous viral antigens. Viral genetic information was identified by cleavage of embryo DNA with restriction endonucleases, electrophoretic separation of the resulting fragments, and identification of bands containing viral sequences by hybridization of the DNA to 32P-labeled viral RNA. Four different chromosomal sites of residence of endogenous viral sequences were identified by this method. These sites were the same as those previously assigned to the endogenous viral loci ev 1, ev 3, ev 4, and ev 5. ev 1 was present in all of the progeny of the cross. ev 3, ev 4, and ev5 were present in various combinations with ev 1. ev 3 cosegregated with the gs+chf+ phenotpye. Cells which did not contain ev 3 but contained ev 1, ev 4, and/or ev 5, did not express detectable levels of viral antigens. We suggest that Gs contains the structural genes for endogenous virus which reside at ev 3 and that these structural genes code for gs and chf in gs+chf+ cells.