Peanut agglutinin, purified by affinity chromatography, agglutinates lymphocytes from mouse, rat, guinea pig, and man only after their treatment with neuraminidase. However, it stimulates only neuraminidase-treated rat and human cells. A similar number of cell surface receptors for peanut agglutinin was found on neuraminidase-treated rat and mouse lymphocytes although the latter cells were not stimulated by the lection. Galactose specifically inhibited the agglutination and stimulation of lymphocytes by peanut agglutinin. Sequential treatment of lymphocytes with neuraminidase and β-galactosidase markedly reduced the response of the cells to stimulation by peanut agglutinin, soybean agglutinin, and galactose oxidase. It is suggested that the same galactosyl residue may be the target for the initial step in triggering lymphocytes by the above mentioned mitogens.