The origin of immunoglobulin on the surface of TDL in the rat has been studied by comparing the binding of purified alloantibodies recognizing the Ig-1a allotype of rat light chain, with that of rabbit antirat Fab antibodies. Both reagents labeled all TDL from rats of the DA strain (Ig-1a) with two categories of cells being detected; one binding 100-2,000 molecules of antibody, the other 10,000-100,000 molecules. These categories were likely to be synonomous with T and B cells, respectively. The [(125)I]antiallotype antibodies did not bind to TDL from rats of the PVG/c strain (Ig-1b). When the binding to TDL from (PVG/c x DA)F(1) animals was studied it was found that allelic exclusion occurred in the heavily labeled cells, but not in the lightly labeled ones. Furthermore, when lymphocytes of one allotype were transferred to irradiated recipients of the opposite allotype and recovered from the TDL or spleen of the recipient 20-30 h later, the immunoglobulin on heavily labeled cells was of the donor type, while that of lightly labeled ones bore the recipient marker. Thus heavily labeled cells (B lymphocytes) had synthesized their own immunoglobulin while lightly labeled cells (T lymphocytes) had acquired theirs passively by adsorption. The class of immunoglobulin on lightly labeled cells was also studied and it was found that [(125)I]anti-IgM antibodies bound to an extent approaching the [(125)I]anti-Fab binding, while [(125)I]anti-IgG(2a+2b) antibodies gave much less binding.