AKR mice, which produce high titers of ecotropic virus, were crossed with NZB mice, which produce titers of xenotropic virus. Spleen, marrow, and lymph node cells of the F1 hybrid produced high titers of ecotropic and xenotropic viruses. However, expression of ecotropic virus by both thymus cells and peripheral T cells of the F1 was severely restricted. Despite simultaneous expression of ecotorpic and xenotropic viruses in F1 spleens, lymph nodes, and marrows evidence for recombinant viruses was not found. Such viruses were also undetectable in the F1 thymuses. The results indicate that a cellular mechanism, present in AKR thymus but lacking in the F1 influences virus expression and the formation of recombinant viruses. This may account for the low incidence of leukemia in the F1 hybrid.