Capsulated mutants of pneumococcus producing a capsule of soluble polysaccharide related immunologically to the C or cell wall polysaccharide of pneumococcus have been isolated from several noncapsulated variants of this organism. The capsular material of these strains reacts with antisera both to homologous strains and to noncapsulated strains of pneumococcus and with human C-reactive protein. C-reactive protein has been shown to give a positive capsular precipitin or Quellung reaction with Cs pneumococcal variants and to agglutinate them.

The genetic locus which determines the production of Cs polysaccharide is situated in a region of the pneumococcal chromosome distinct from that controlling normal capsular polysaccharide synthesis. Binary and ternary capsulated pneumococci, one of the capsular components of which is Cs polysaccharide, have been isolated following DNA-mediated transformation.

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