We have studied glycosyltransferase activities in human lymphocytes stimulated with the plant mitogens E-phytohemagglutinin or concanavalin A (Con A) and have compared the results with activities found in resting lymphocytes. Compared to resting lymphocytes, Con A-stimulated lymphocytes possess an enhanced capacity to transfer the sugars, sialic acid, galactose, and N-acetylglucosamine from their respective nucleotide donors to both endogenous cellular acceptors and added exogenous glycoprotein acceptors. The enhanced glycosyltransferase activity induced by Con A is not inhibited by puromycin despite effective inhibition of de novo protein synthesis, indicating that synthesis of new glycosyltransferase enzymes is not necessary for the observed increases in glycosylation activity. By contrast to the findings in Con A-stimulated cells, the corresponding glycosyltransferase activities of E-phytohemagglutinin-stimulated lymphocytes do not differ from those of unstimulated lymphocytes. These data indicate that individual plant lectins have different effects on the biosynthesis of complex saccharides by cultured human lymphocytes.