Thirty-three species of marine algae belonging to Rhodophyta, Phaeop hyta and Chlorophyta from the Fujian coast were examined for agglutinins with differe nt animal and human erythrocytes. Protein extracts from 26 species were active against at least one type of the erythrocytes tested. There were 3 species (Grateloupia imbricata, Ishig e foliacea and Entermorpha prolifera) whose extracts could agglutinate all the erythrocytes used. The lowest protein concentration required to produce erythrocyte agglutination varied rema rkably, from 3.1 μg/ml to 500 μg/ml . The strongest activity was found in the agglutinatio n of rabbit erythrocytes by Gloiopeltis furcata extract. Inhibition assays performed wit h nine mono- and bisaccharides indicated that agglutinations of rabbit erythrocytes by extracts o f 7 species were inhibited by one or more types of the sugars assayed. The agglutinating act ivity shown by extracts of most species was not affected when the test solution was heated t o 90℃, but was lost at 95℃-100℃. A few extracts lost their activity at 60℃, 65℃ and 75 ℃, respectively.