A naturally occurring hemagglutinin was detected in the serum of the freshwater crab, Paratelphusa jacquemontii (Rathbun). Hemagglutination activity with different mammalian erythrocytes suggested a strong affinity of the serum agglutinin for horse and rabbit erythrocytes. The most potent inhibitor of hemagglutination proved to be bovine submaxillary mucin. The lectin was purified by affinity chromatography using bovine submaxillary mucin‐coupled agarose. The molecular mass of the purified lectin was 34 kDa as determined by SDS/PAGE. The hemagglutination of purified lectin was inhibited by N‐acetylneuraminic acid but not by N‐glycolylneuraminic acid, even at a concentration of 100 mm. Bovine submaxillary mucin, which contains mainly 9‐O‐acetyl‐ and 8,9 di‐O‐acety‐N‐acetyl neuraminic acid was the most potent inhibitor of the lectin. Sialidase treatment and de‐O‐acetylation of bovine submaxillary mucin abolished its inhibitory capacity completely. Also, asialo‐rabbit erythrocytes lost there binding specificity towards the lectin. The findings indicated an O‐acetyl neuraminic acid specificity of the lectin.