High m.w. glycoproteins in the range of 170 and 210K represent major cell surface components of murine lymphocytes. The expression of such glycoproteins has been characterized for thymocytes and peripheral T and B lymphocytes in BALB/c mice using surface and biosynthetic labeling, nonionic detergent extraction, and immunoprecipitation with antibodies directed against antigenic determinants expressed on the cell surface of a murine T cell lymphoma. Immunoprecipitates were then resolved by 1-dimensional and 2-dimensional gel electrophoresis. Two groups of antigenically distinct glycoproteins have been defined: 1) a 170K component present on thymocytes and peripheral T lymphocytes but undetectable on B lymphocytes; this glycoprotein has been termed T 170 to suggest its potential use for distinguishing T from B lymphocytes; 2) a higher m.w. glycoprotein bearing antigenic determinants common to T and B cells but expressed as one 180K glycoprotein on thymocytes, 2 glycoproteins of 180 and 190K on peripheral T lymphocytes, and as one 210K glycoprotein on B lymphocytes. The relationships of these glycoproteins with the other known lymphocyte surface markers of high m.w. is discussed.