Human lymphocytes from thymus, spleen, and blood were stimulated by concanavalin A, phytohemagglutinin or pokeweed mitogen to incorporate radioactive thymidine and various carbohydrates. Peripheral-blood lymphocytes were also activated by alloantigen in a mixed lymphocyte reaction. Incorporation of thymidine and carbohydrates was parallel in stimulated cells. Carbohydrate-labelled cells were extracted with Triton X-100 buffer and the extracts subjected to sodium dodecylsulfate/polyacrylamide gel electrophoresis and subsequent autoradiography. From a comparison of the resulting glycoprotein patterns the following conclusions could be drawn. (a) The same population of thymocytes responds to all three mitogens. (b) Pokeweed-mitogen-stimulated blood lymphocytes are similar to this mitogen-responsive thymocyte population. (c) The glycoprotein pattern of pokeweed-mitogen-responsive spleen cells shows two characteristic glycoproteins, GP 50 and GP 78, and resembles that of murine B cells. (d) Whereas concanavalin A and phytohemagglutinin activate identical subpopulations of T cells in the spleen, this is not the case in the blood. (e) Concanavalin-A-activated and alloantigen-activated blood lymphocytes express a glycoprotein GP 185 which is not found on phytohemagglutinin-responsive cells. (f) The kinetic relationship during alloantigen stimulation in a mixed lymphocyte reaction between the appearance of glycoprotein GP 185 and subsequent maximal killing activity in an assay using release of 51Cr makes it likely that glycoprotein GP 185 is a marker for cytotoxic T lymphocytes.