The yield of purified type III polysaccharide of group B Streptococcus was significantly improved by modification of the growth medium. Culture of organisms in standard Todd-Hewitt broth resulted in acid accumulation during the exponential phase of growth and poor yield of type III polysaccharide when extracted from cells by washing with neutral buffer solution. By increasing the buffering capacity of the broth medium, acid accumulation was prevented, and the number of viable cells was increased at the stationary phase of growth. Further, by increasing the concentration of glucose in the buffered medium, the yield of type III polysaccharide was increased two to three times. Electron microscopic investigations of cells grown in the modified broth medium demonstrated a thicker microcapsule than was found in organisms grown in standard broth.