By means of careful neutralization of polyvalent PC phage acting on B. coli, B. dysenteriae Shiga, and B. dysenteriae Flexner, it has been possible to neutralize the “valences” for the latter two organisms and to secure a monovalent phage acting on B. coli only. This monovalent phage promptly regains its polyvalence when it has acted upon B. coli.

It has been shown that the valence for B. dysenteriae Shiga is neutralized at a faster rate than the coli valence. The difference in rates is probably not due to the fact that it may take a different minimum number of antibody molecules to neutralize a valence for B. coli than that for B. dysenteriae Shiga.

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