We have previously surveyed marine algae for hemagglutinins and detected twenty active species. Here, for elucidating the properties of the algal agglutinins, we examined the agglutinins of seven active species for agglutinating activities toward other biological cells besides erythrocytes, sugar-binding specificities, and effect of heat, PH and divalent cations on then hemagglutinatingactivities, using the crude extracts or partially purified agglutinins. As a result, we found that most of the algal agglutinins agglutinated unicellular blue-green algae and mouse tumour cells FM3A, and two to three agglutinins agglutinated yeasts and bacteria. Sugar-binding specificities of the algal agglutinins were different from those of agglutinins from other sources; most hemag-glutinating activities were strongly inhibited by many glycoproteins tested and glycopeptide-fractions from fetuin, but not generally by monosaccharides, suggesting that algal agglutinins bind specifically to some complex carbohydrates. Algal agglutinins were not affected by addi-tion of divalent cations such as Ca²⁺, Mg²⁺ and Mn²⁺, unlikely agglutinins from other sources, and were relatively stable at the wide range of temperature and pH. These data indicate that marine algae may provide the available sources for agglutinins with unique properties.