Departments of Pathology and Medicine, The Ohio State University, Columbus, Ohio 43210 Received September 26, 1985; Revised Manuscript Received January 13, 1986 abstract: The effects of phytohemagglutinin (PHA) stimulation on the activities of sialyltransferase 1 (SAT-1), and sialyltransferase 3 (SAT-3), in human lymphocytes were investigated in vitro. For SAT-1 and SAT-3, respectively, the apparent Km values with variable CMP-NeuAc concentrations were 0.19 and 0.015 mM and with variable LacCer were 0.075 and 0.17 mM. Progressive increases in the activities of SAT-1 and SAT-3 were detected in lymphocytes stimulatedwith PHA, whereas no increase was observed in control lymphocytes incubated in culture medium alone. These increased activities occurred within 18-36 h of incubation and preceded optimum lymphocyte proliferation. Intact lymphocytes were needed for the lectin-stimulated increase of sialyltransferase activities because neither concanavalin A nor phytohemagglutinin added to the broken cellpreparation modulated SAT-1 activity. The glycolipid products formed as a result of these enzymatic reactions in the presence of endogenous and exogenous acceptors were tentatively identified by thin-layer chromatography and autofluorography. Theaddition of exogenous LacCer to theSAT-1 assay resulted in the radiolabeling of a small amount of ganglioside GMlb (3.4%), but GM3 was the major labeled product (96%). When GgOse4Cer was added to the SAT-3 assay, 32% GM3 and 24.6% GMlb were detected while 44% consisted of glycolipids not labeled in assays performedwithout exogenous acceptors. Of the radioactivity transferred to endogenous acceptors, 81.3% was in GM3 and 14.6% in GMlb. These results demonstrate that the modulation of sialyltransferase activity occurs earlier than cellular activation.