An encapsulated strain of Escherichia coli has been isolated which is hemolytic, pathogenic for mice, and which has served to illustrate further evidence of heterogenetic specificity. The relationship appears to be limited to the serological reactions between the colon organism and Type I antipneumococcic horse serum. Type I antipneumococcic rabbit serum failed to agglutinate the organism and no reactions occurred with Types II and III antipneumococcic horse serums, normal horse serum, and a variety of other immune horse serums. Serum from rabbits immunized with the colon bacillus agglutinated the homologous organism and precipitated its soluble substance, but failed to cause agglutination of Type I pneumococci or to precipitate Type I pneumococcic polysaccharide. The evidence indicates a connection somewhat analogous to that between Type II pneumococcus and Type B Friedländer's bacillus.

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