The cross-reactivity and chemical characterization of the nongroupable streptococcal and pneumococcal group 19 polysaccharides (PS) have been studied. Extensive cross-reactions were observed between capsular PSs of streptococcal strains 14636/74, 4907, 4731 and pneumococcal type 19F and 19A antisera. Streptococcal 14636/74 PS had an identical composition to that of pneumococcal 19F PS. Type 19F and 14636/74 PS were composed of equimolar amounts of rhamnose, glucose, N-acetyl mannosamine, and phosphorus. The capsular PS of strains 4731 and 4907 contained rhamnose, glucose, ribose, N-acetyl mannosamine, and N-acetyl glucosamine in different molar ratios. Extensive immunologic reactivity was observed between the 19F and 14636/74 PS, as determined by light scattering rate nephelometry, passive immune hemolysis, and precipitin reaction. There was an identity reaction by immunodiffusion between type 19F and 14636/74 PS when reacted with rabbit antiserum against either organism. Biochemical studies showed that strain 14636/74 was not a pneumococcus, because it was optochin resistant, was bile insoluble, did not possess the C-carbohydrate antigen common to all pneumococci, and produced neither pneumolysin nor IgA protease. Furthermore, it grew in comparatively simple media in contrast to the complex nutritional requirements of pneumococci. The 13C-NMR spectra of the 19F and 14636/74 PS were identical. These two capsular PS can, therefore, be considered identical.