1. By partial acid hydrolysis a specific carbohydrate may be isolated from gum arabic (gum acacia). This carbohydrate is comparable in its precipitating activity for Type II (and Type III) antipneumococcus serum with the bacterial soluble specific substances themselves.
2. On hydrolysis this fraction yields galactose and two or more complex sugar acids, one of which appears to be a disaccharide add comparable with those isolated from the specific polysaccharides of the Type III pneumococcus and the Type A Friedländer bacillus.
3. The significance of these findings is discussed.