The paper reports a study of a virulent, S-producing strain of Pneumococcus which is immunologically related to, but not identical with typical strains of Type III pneumococcus. In a potent anti-Type III serum, the relationship of this strain to typical Type III strains appears to be about the same as the relationship of Avery's Subgroup Type II strains to typical Type II. But a more pronounced distinction is evident in the antiserum produced by immunization with the strain related to Type III. This antiserum contained antobodies specifically reactive with typical Type III bacteria as well as antibodies reactive with the homologous strain, while anti-Subgroup Type II immune sera are devoid of antibodies reactive with typical Type II pneumococci.

The result of absorption experiments were the same as those usually obtained with immunologically related, but not identical bacteria. The failure of reciprocal absorption and the marked variations in the relative potencies of the antiserum from different individual animals might be presented as presumptive evidence that two different anti-S antibodies are contained in Type III immune horse serum.

The theoretical significance of virulent pneumococci which are related to but not identical with the "fixed" types, is discussed from the standpoint of their importance in the biological classification of the Pneumococcus group.

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