Thymocytes, bone marrow cells, and their derived T and B cell populations were examined for the presence of Ig by the cell surface radioiodination technique. Both IgM and IgG were identified on bone marrow cells. Thymocytes and T cells had no detectable cell surface Ig. Radiolabeling of mixtures of B cells and thymocytes suggest that the method may detect as little as 250 molecules of Ig per cell. Based on these findings, we suggest that the T cell receptor for antigen is not a conventional tetrameric Ig
