A non-antigenic mucoid polysaccharide similar to that described by Kendall, Heidelberger, and Dawson was isolated from group C hemolytic streptococci. A simple method for its quantitative estimation is described. By means of this quantitative method, as well as by the direct isolation of the carbohydrates, the size and persistence of capsules in young cultures of various strains have been related to the non-antigenic mucoid polysaccharide. Similarly, invasiveness in different strains has been related to the mucoid polysaccharide. Data indicating the non-haptene nature of this material are presented. An autolytic process, accelerated by bile, is involved in the loss of capsular material from young streptococci.