The effects of concanavalin A and succinylated concanavalin A on the transformation of mouse splenic lymphocytes, and on early biochemical events in the transformation, were compared. 1. The transformation of lymphocytes is biphasic with respect to concanavalin A concentration with optimal activation at about 1 microgram/ml. Activation by succinyl concanavalin A is not biphasic over a range of lectin concentration of 1--16 microgram/ml. 2. In intact lymphocytes cultured for 4 h, the enzyme Acyl-CoA:1-acylglycero-3-phosphocholine O-acyltransferase (EC 2.3.1.23) was not activated by concanavalin A but was inhibited at all concentrations tested, and was about 60% inhibited at 16 micrograms concanavalin A per ml. Succinyl concanavalin A gave little or no inhibition at similar concentrations. 3. Lymphocytes become committed to divide while their acyltransferase activities are markedly inhibited by concanavalin A. 4. The inhibition of acyltransferase by concanavalin A can be lifted by displacing the lectin from the cells by alpha-methylmannoside. Lowered enzyme activity is not caused by cell agglutination or by direct cross-linking of lectin receptors. It is unlikely that the inhibition of acyltransferase is due to indirect cross-linking via the cytoskeleton since colchicine did not reverse the inhibition. 5. The inhibition of acyltransferase and the reduced stimulation of transformation by higher levels of concanavalin A appear to be due to hydrophobic interaction of the lectin with the plasma membrane, as shown by liposome aggregation studies.