AbstractCell surface molecules of cultured human lymphoid cells were selectively labeled by lactoperoxidase‐catalyzed iodination and examined by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Two major iodinated species with apparent mol. wts. of 27 000 and 35 000 daltons were detected on autoradiographs of the labeled proteins of human lymphoid cell lines believed to be of thymus‐independent (B) cell origin. Neither molecule was detected on putative thymus‐dependent (T) lymphoid cell lines. Metabolic labeling studies showed that both molecules are glycoproteins. Rabbit antisera against cultured B lymphoid cells made specific by absorption for B cells reacted with several labeled species from iodinated B cells including the 27 000 and 35 000 mol. wt. glycoproteins. These molecules were also detected on tonsillar lymphocytes but not on peripheral blood lymphocytes. Reciprocal absorption with B cells of rabbit antisera against cultured T cells gave antisera specifically cytotoxic for T cells. However, these sera did not precipitate iodinated proteins from Nonidet P‐40 lysates of
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