A mitogenic lectin that has a carbohydrate-binding specificity similar to that of concanavalin A (Con A) can be isolated from pea seeds. Chemical modification (succinylation, acetylation, or treatment with the diazonium salt of sulfanilic acid), changes its biological properties. In mitogenic stimulation of mouse spleen cells and in hemagglutination, the differences between the chemically modified pea lectin and the native molecule are similar to those observed between succinyl-Con A and native Con A. However, whereas chemical modification converts tetrameric Con A to a dimeric molecule, similar treatments of the pea lectin do not change its quaternary structure. The results of binding studies of the pea lectin and its derivatives to mouse spleen cells suggest that the differences in biological activities may be explained by a reduction in binding affinity of the pea lectin for glycoproteins on the spleen cell surface after chemical modification.