Lectin agglutination and cell surface galactosyltransferase (EC 2.4.1.67; 1-O-alpha-D-galactosyl-myo-inositol:raffinose galactosyltransferase) enzyme activity have been studied with thymus and spleen lymphocytes of neonatal rats. Thymus lymphocytes were more agglutinable by concanavalin A than by wheat germ agglutinin, whereas spleen lymphocytes were more agglutinable by wheat germ agglutinin than by concanavalin A. Thymus lymphocytes, but not spleen lymphocytes, of neonatal rats could be blast transformed by concanavalin A. Cell surface galactosyltransferase activity was present on both types of lymphocytes, but was greatly increased on thymus cells after blast transformation. The differences in lectin agglutination suggest a difference in the surface membranes of thymus and spleen lymphocytes. The increase in cell surface galactosyltransferase activity with blast transformation of thymus lymphocytes may be related to the exteriorization of the Golgi apparatus into the plasma membrane.