Four cloned continuously cultured mouse T lymphoma cell lines, WEHI-22.1, WEHI-7.1, S49.1, and EL-4.1, were examined for immunoglobulin biosynthesis and the presence of immunoglobulin on the cell surface. Incorporation of [-3H]leucine into cellular proteins followed by serological analysis showed that immunoglobulin constituted between 0.1 and 1.1 percent of protein synthesized by the different cell lines during a 6-hr period. Under the same conditions cultured cells of nonlymphoid origin, the mastocytoma P-815 X-2.1, did not synthesize any detectable immunoglobulin. Lactoperoxidase-catalyzed radioiodination was used to label proteins on the surface of viable lymphoma and mastocytoma cells. Although the lymphoma lines lacked immunoglobulin as assessed by fluorescent antibody staining, immunoglobulin was detected in surface proteins of all four lymphoma lines. Estimates of the number of immunoglobulin molecules on the cell surface were 1.1 times 10-4/cell for S49.1 and EL-4.1, 1.7 times 10-4 for WEHI-7.1, and 4.3 times 10-4 for WEHI-22.1. Electrophoretic mobilities in sodium dodecyl sulfate polyacrylamide gel indicated that intact cell surface immunoglobulin was slightly larger than IgG, and on disulfide bond reduction to dissociate into two components, one with the mobility of serum immunoglobulin light chain, the other with a mobility similar to that of mu heavy chain. The heavy chain from the T lymphoma cells possessed an apparent molecular weight of about 65,000 compared with 70,000 for mu chain, although both chains shared antigenic determinants characteristic of mu chains. These findings are interpreted as support for other reports that T lymphocytes carry immunoglobulin on their surface and as direct evidence that thymus-derived lymphoid cells synthesize an immunoglobulin resembling the 7S subunit of IgM.