Plasma membranes prepared from rat thymocytes were used to immunize rabbits and after 6 months the resulting sera had cytotoxic titers of 1:1280 to 1:2560 for rat thymocytes. With the lactoperoxidase-catalyzed iodination reaction the thymus plasma membranes were labeled with 125Iodine, solubilized in non-ionic detergent, and immuno-precipitated with the cytotoxic rabbit anti-rat thymus sera. These sera bound 3.5 to 4.4% of the total acid insoluble counts while control sera bound 0.6%. Subsequently, the precipitates were solubilized, reduced, and characterized on SDS containing polyacrylamide gels. The labeled antigenic component proved to have a m.w. ∼45,000. The same antigenic material could be labeled by in vitro incubation of rat thymus cells in 14C amino acids. Absorption studies suggested that this antigenic membrane constituent was shared by all lymphoid cells and represented a major immunodominant area on lymphocyte surfaces during heterologous immunization.