The oncornavirus related proteins associated with the surface of normal and malignant thymocytes were studied. Three virion-associated proteins (gp69/71, p45, p30) were associated with lymphoma cells from about 70% of the tumors studied. Two virion-associated proteins (gp69/71 and p45 were associated with normal thymocytes form some but not all strains of mice. In gp69/71- mice, conversion to the gp69/71+ phenotype accompanied leukemogenesis. An interesting difference in the apparent molecular size of virus related antigens of the 70,000 dalton size class was detected in lymphoma cells present in involved spleens as compared to involved thymuses. Mice infected as neonates with Scripps leukemia virus make antibody to gp69/71 and some make antibodies to molecules associated with the surface of their own tumors. The significance of the restricted presence of antigens coded for by the viral genome to the surface of some differentiated cells is discussed in reference to (a) the relationship between virion, leukemia associated, and differentiation dependent markers, and (b) the possible consequence to the host of having similar antigenic determinants on three independent structures with replicative potential (virus, normal thymocytes, and tumor cells).