Studies on the maturational lineages of thymic lymphocytes have revealed several subclasses which are distinguishable on the basis of cell size, topographic distribution within the thymus, DNA synthetic and mitotic activity, migratory behavior, and other properties. Strain C57BL/Ka mice were inoculated with radiation leukemia virus at different concentrations, and tissues were removed at defined intervals. Sequential sections were analyzed for virus-specific cytoplasmic antigen expression, for morphological evidence of neoplastic transformation, and for alkaline phosphatase activity. The first detectable sign of MuLV infection was the focal appearance of cytoplasmic viral antigens in cells of the outer thymic cortex, followed by coalescence of such foci and, several weeks later, by the appearance of morphologically transformed and alkaline phosphatase-positive cells, again often focally distributed in the outer thymic cortex. These observations strongly suggest that the large, mitotically active cells of the outer thymic cortex are the principal source of target cells for both productive infection and subsequent lymphoma induction by the virus.