1. Agglutination and precipitation by the specific substance of Streptococcus viridans are parallel phenomena. Separate specific substances have been extracted from strains which are distinct by ordinary serological tests. Preliminary chemical examination indicates that the specific substances may be complex carbohydrates.

2. A close relationship between nucleoproteins from different strains of Streptococcus viridans is suggested by the existence of a certain amount of cross-precipitation and a larger degree of cross-complement fixation. But the occurrence of stronger reactions with homologous nucleoproteins than with heterologous indicates that there is some degree of individual difference in proteins from separate strains.

3. Two distinct antibodies are present in the sera antibacterial for Streptococcus viridans: one of high titer implicated in the parallel phenomena of agglutination and precipitation by the soluble specific substance, the other usually of low titer and involved in precipitation by nucleoproteins but probably little, if at all, in agglutination.

The significance of these results obtained from the study of antibacterial sera will be considered in the general discussion of the antigenic components of Streptococcus viridans after the results from the study of antinucleoprotein sera have been presented in the succeeding paper.

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