T lymphocytes at various stages of maturation and differentiation have been isolated by cellular fractionation procedures and characterized by cell surface markers and functional assays, The cell surface glycoproteins of the various T-cell preparations have been selectively radiolabeled by the galactose oxidase-tritiated sodium borohydride technique and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography. Details are presented on the appearance of a new cell surface glycoprotein (T 145), present on immunocompetent T lymphocytes after activation by either major histocompatibility complex alloantigens or by concanavalin A. The intensity of T 145 expression on T lymphoblasts is shown to be directly correlated in time and extent to the levels of cytotoxicity generated in a variety of T-cell activations. Specific enrichment procedures of purified populations of mixed leukocyte culture blasts have shown Ly 1(+)2(-) blasts to be T 145(-) and Ly 1(-)2(+) blasts to be strongly T 145(+). Similar enrichment procedures on normal peripheral T cells have failed to reveal any significant expression of T 145 on a highly enriched population of Ly 1(-)2(+) T cells, Further studies on the stability of T 145 expression after induction have shown it to be a more permanent-type differentiation structure whose expression is clearly not linked to the blast stage of activation. T 145 would thus appear to represent a membrane glycoprotein whose exclusive expression on T lymphoblasts is further restricted to a defined group of cells endowed with cytolytic activity and bearing the Ly phenotype Ly 1(-)2(+).