The correlation of murine leukemia virus (MuLV) infection and leukemogenesis was examined by XC plaque assay in W/F rats neonatally infected with the rat-adapted Gross virus (RAGV). Thymic lymphomas developed in 100% of infected rats, with a mean latent period of 64 days. The virus infectivity was first detected in the thymus as early as 15 days after the virus infection; the titer steadily increased thereafter until the development of thymic lymphoma. No other tissues of virus-infected rats showed virus infectivity until the development of localized thymic lymphoma, though sera of a few rats showed low infectivity titers. In rats with generalized leukemia, however, high titers were detected in the sera and leukemic tissues. In untreated controls, all tissues tested invariably showed negative titers for MuLV infectivity at any age up to 7 months after birth. The results indicated that those tissue sites were common to both RAGV infectivity and the leukemogenic process, with the primary involvement of the thymus, and that the appearance of RAGV infectivity in various tissues represented the expression of the oncogenic genome of RAGV.