Development of the in vivo transplantable D1-DMBA-3 mammary tumors results in an alteration of several cytokines in the host, and IFN-γ is one of the most severely downregulated. Notably, the thymuses of these mice display a profound atrophy that is associated with a severe depletion of CD4⁺8⁺ thymocytes. Investigations into the possible mechanisms that lead to this thymic atrophy revealed that the levels of proliferation assessed by in vivo labeling with 5' -bromo-2'-deoxyuridine (BrdU) were similar in control and tumor-bearing mice. However, our studies implicated a modest increase in apoptosis, coupled with an arrest at the triple negative stage of differentiation in the thymic hypocellularity in tumor bearers. We have transfected the DA-3 mammary tumor cell line, derived in vitro from the in vivo D1-DMBA tumors, with the IFN-γ gene and showed the production of high levels of IFN-γ protein by the transfected cells. Inoculation of hosts with IFN-γ transfected cells 4 days prior to challenge with the D1-DMBA-3 tumor resulted in a blockage of the thymus involution in these mice. In contrast, using in the same protocol untransfected DA-3 cells, the progressive atrophy observed in animals with D1-DMBA-3 tumors was observed. These results suggest that the lack of IFN-γ may be an important factor in the thymic atrophy that occurs during mammary tumorigenesis.