The aim of this study was to examine the heterogeneity of membrane proteins of several B and T lymphoblast cell lines and activated normal T lymphocytes using SDS gel electrophoresis of [35S]methionine metabolically labeled cells. In part, we hoped to test the hypothesis that human lymphocytic malignancies are “frozen” representatives of various stages in the differentiation paths of normal lymphoid cells. The membrane protein patterns of Burkitt's lymphoma cell lines were very similar to each other and to those of EBV-transformed B lymphoblastoid cell lines from healthy persons. The American Burkitt's lymphoma cell line Ramos lacked p29, 34 (HLA-DR) antigen, as was confirmed with immunoprecipitation studies of this complex and of p44, 12 (HLA-A, and -B antigens and β2-microglobulin) from each of the cell lines. Heterogeneity in the electrophoretic mobility of HLA-DR antigen was apparent, reflecting size or charge variations. Small degrees of membrane protein heterogeneity among the T lymphoblast cultured cell lines was also evident. Proteins of mol. wt 90,000, 75,000, 18,000 and 16,000 were more heavily labeled on some cell lines than on others. This heterogeneity is consistent with the hypothesis that these tumors might be derived from cells of varying maturational stages or from different subsets of T lymphocytes. In contrast to the T lymphoblast lines, Con A-activated T lymphocytes on each successive day of stimulation expressed dramatically different membrane proteins. Proteins of mol. wt 30,000 and 36,000 were found on day 2 cells; mol. wt 55,000 proteins were dominant on days 1, 2 and 3; a group of proteins of mol. wt about 120,000 was present on cells surviving a lytic process on day 4. Although many proteins were shared between Con A blasts and cultured T cell lines, the absence from the cultured lines of proteins which were dominantly synthesized by mitogen-activated lymphoblasts was consistent with the view that long-term cultured lymphoblasts represent a more primitive, undifferentiated, immature T-cell population, while the changes seen with Con A activation represent metabolically controlled changes consequent to activation of one or more subsets of mature T cells. While many proteins were shared among membrane proteins of T lymphoid tumors and mitogen-activated, normal T lymphocytes, the differences were so striking that simple parallelisms between these lymphoid tumors and activated sets of normal lymphocytes could not be proposed.