Thirty-one strains representing eight species and four genera of the molluscan family Planorbidae were surveyed for the presence of hemagglutinins and hemolysins. Extracts prepared from albumen-glands and egg-masses, as well as hemolymph, were assayed by a micro-hemagglutination technique in which human erythrocytes were used as receptors. 2. Hemagglutinins and hemolysins for human erythrocytes were not uniformly present in all members of the family and detection of these substances depended, in part, on the material tested. 3. Neither heating to 56 degrees C, dialysis, storage at -20 degrees C for up to a year, nor repeated freezing and thawing appeared to effect the agglutinating activity of egg or albumen-gland extracts. 4. Inhibition of the agglutinating activity of Helisoma hemolymph could be accomplished with N-Acetyl-D-Galactosamine, but this sugar had no effect on the activity of Biomphalaria agglutinins. 5. There is evidence to suggest that the source and nature of agglutinins from Helisoma and Biomphalaria species are different. 6. Lectins may be of some value in characterizing snail populations and as an aid in the taxonomic discrimination of species.
