Hemagglutinating activity can be identified in the plasma of different species of murrel fish. This activity may be divided into four types according to their agglutinability towards erythrocytes from different sources. Type I plasma agglutinates human blood group A erythrocytes, type II can agglutinate neuraminidase treated human A B O erythrocytes, type III shows no agglutinating activity towards human erythrocytes, while type IV agglutinates human erythrocytes non-specifically. All of them bind to DEAE-cellulose but elute out by different salt concentrations. Type IV plasma is found to be a combination of three separate hemagglutinins, which are separable by sequential binding to human A B O erythrocytes. Blood group A specific lectin activity is purified from this plasma using formalinised A group erythrocytes. The apparent homogeneity of this purified lectin is established by polyacrylamide gel electrophoresis, isoelectric focusing and immunodiffusion. This agglutinin is antigenically identical with that isolated from type I plasma by affinity chromatography on N-acetyl-D-galactosamine coupled to epoxy-activated cellulose column. Their molecular weights are also found to be identical (Mr 140,000) in polyacrylamide gel electrophoresis, having two identical subunits. Forssman glycolipid (0.03 mM) was found to be the most potent inhibitor of agglutination, although Gal beta 1-3 GalNAc (0.09 mM) is also a good inhibitor. Exhaustive dialysis of the purified lectin (hemagglutinin) against EDTA denatures it irreversibly by dissociating it to its subunit structure. Thus human A group agglutinating activity isolated from type I and type IV plasma are identical.