10.22376/ijpbs.2019.10.1.p1-12 Volume 5 Issue 3 2014 (July- September) A naturally occurring hemagglutinin (HA) was detected in the serum of freshwater fish lessThan i greaterThan Catla catla lessThan /i greaterThan using mammalian erythrocytes (RBC) as indicator cells. The serum gave the highest HA titer with human B RBC. In cross adsorption tests, this RBC type completely adsorbed all HA activities from fish serum. An analysis of the physico-chemical properties of HA showed it to be specifically dependent on the presence of Ca lessThan sup greaterThan 2+ lessThan /sup greaterThan for its activity, irreversibly sensitive to EDTA, stable between pH 6.0 to 8.0 and the thermal stability stable between 10 to 30 lessThan sup greaterThan 0 lessThan /sup greaterThan C. Further studies demonstrated that the HA is precipitable by Ammonium sulphate (25, 50 and 75%) but 10% TCA did not show precipitate. The Hemagglutinin - inhibition (HAI) assays performed with 25 carbohydrates revealed that the serum HA was specific for inhibited by GlcNAc, GalNAc, laminarin and lipopolysaccharide ( lessThan i greaterThan Escherichia coli lessThan /i greaterThan ). Thus this agglutinin to be unique among all the known fish agglutinins S.S.JAYARAJ hemagglutinin; serum; freshwater fish; Catla catla. 145-155