Only a few animal phyla have been screened for the presence and distribution of lectins. Probably the most intensively studied group is the mollusk. In this investigation, 22 species from 12 families of tropical sponges collected in Los Roques National Park (Venezuela) were screened for the presence of lectins. Nine saline extracts exhibited strong hemagglutinating activity against pronase-treated hamster red blood cells; five of these reacted against rabbit red blood cells, four with trypsintreated bovine red blood cells, and five with human red blood cells regardless of the blood group type. Extracts from the three species studied from genus Aplysina (archeri, lawnosa, and cauliformis) were highly reactive and panagglutinating against the panel of red blood cells tested. The lectins from A. archeri and A. lawnosa were purified to homogeneity by ammonium sulfate fractionation, affinity chromatography on p-aminobenzyl-␤-1-thiogalactopyranoside-agarose, and gel filtration chromatography. Both lectins exhibited a native molecular mass of 63 kDa and by SDS-polyacrylamide gel electrophoresis under reducing conditions have an apparent molecular mass of 16 kDa, thus suggesting they occur as homotetramers. The purified lectins contain 3-4 mol of divalent cation per molecule, which are essential for their biological activity. Hapten inhibition of hemagglutination was carried out to define the sugar binding specificity of the purified A. archeri lectin. The results indicate a preference of the lectin for nonreducing ␤-linked D-Gal residues being the best inhibitors of red blood cells binding methyl-␤-D-Gal and thiodigalactoside (Gal␤1-4-thiogalactopyranoside). The behavior of several glycans on immobilized lectin affinity chromatography confirmed and extended the specificity data obtained by hapten inhibition.In animals, only a few phyla have been screened for the presence and distribution of lectins. In particular, the number of lectins that have isolated from invertebrate organisms is quite small as compared with the great variety of lectins isolated from plant origin and have been limited to those partially characterized from mollusks and crustaceans. Since the discovery of hemagglutinins in sponges by Dodd et al. (1), there have been some reports on the partial characterization (serological and immunoelectrophoretic properties) of lectins from the oldest multicellular animals (2, 3) from the Mediterranean Sea or Japan (4 -8). However, their properties and specificities have not been clearly defined. In this report, we present data on an investigation undertaken to search for novel lectins in marine organisms, which could show unique properties. In addition, we describe the purification and characterization of the lectins present in two species of tropical sponges, Aplysina lawnosa and Aplysina archeri. We present information on the nature and specificity of their combining site as examined by hapten inhibition experiments and affinity chromatography.