In the present work, we attempted to detect immunoglobulin on or in thymocytes, various thymus-derived lymphocytes, spleen cells, and B lymphocytes, using cell-surface radioiodination as well as biosynthetic labeling procedures. The labeled proteins were precipitated by antibodies directed against mouse immunoglobulin chains and the precipitates were analyzed by radioautography after Na dodecyl sulfategel electrophoresis (9), a sensitive procedure with high resolving power. MATERIALS AND METHODSLactoperoxidase-Catalyzed Radioiodination of Cell-Surface Proteins. Thymocytes and spleen-cell suspensions were prepared as described (3), and used only when more than 95% of the cells were viable, as judged by trypan blue exclusion. Iodination was performed by the method of Marchalonis et al. (10) usually on 108 cells with 1 mCi of carrier-free Na-125I (80-140 mCi/ml, Amersham), 100 ,g of lactoperoxidase (Sigma, St. Louis, Mo.), and 30 ,l of 8.8 mM H202. After a 5-min incubation at 300, the cells were centrifuged at 4°. They were washed once with 5 mM L-cysteine -HCl in phosphatebuffered saline (pH 7.4), twice with 5 mM L-cysteine-HCl-10 mM KI in phosphate-buffered saline, and once with 10 mM KI in phosphate-buffered saline. Trypan blue exclusion showed no loss of viability. Three procedures were used to solubilize the labeled cell-surface proteins. Bethesda, containing 5% fetal-calf serum), followed by extensive dialysis against phosphate-buffered saline of the culture medium containing released labeled proteins. The extracts obtained after the various procedures were analyzed for total, as well as trichloroacetic acid-precipitable, radioactivity with a Packard Autogamma spectrometer. Usually 10-30% of the radioactivity was not acid-precipitable.Abbreviations: B lymphocyte, bone marrow-derived, thymusindependent lymphocyte; T lymphocyte, thymus-dependent lymphocyte.2879