Hydrazinolysis of blood-group A mucopolysaccharide of hog gastric mucus resulted in the considerable cleavage of the substance which accompanied the concomitant extensive degradation of sugar components of the mucopolysaccharide.The extent of the degradation of the sugar components and the cleavage of the mucopolysaccharide were enhanced by the increase in the amount of hydrazine and also by the repeated hydrazinolysis.Of the sugar components in the mucopolysaccharide, sialic acid was completely destroyed by the hydrazinolysis and galactosamine was the most labile against the above treatments among the four main sugars.Small amounts of 2,5-anhydrohexoses, pentoses (lyxose and arabinose) and some other unknown products were found in the hydrazinolyzates of the mucopolysaccharide.Serological specificity of the blood-group A mucopolysaccharide disappeared completely after the first treatment.The mechanisms of the degradation of the sugar components and of the cleavage of the mucopolysaccharide were discussed.