1. A rapid method for the roughly quantitative estimation of mucoid polysaccharide in hemolytic streptococci has been described.
2. Using this method, about 94 per cent of strains from moderate or severe streptococcal infections in man have been found to produce mucoid polysaccharide in greater or less amount. In a group of streptococci from normal throats only about 8 per cent produced this substance, all of the producers falling into Lancefield's group A.
3. Of the Lancefield group A strains from both normal and infected sources, 92 per cent showed the presence of mucoid polysaccharide in culture dilutions of 1:10 or higher.
4. The probable significance of the mucoid polysaccharide in streptococcal virulence is indicated.