The paper reports an experimental comparison of the antibody responses of mice and of rabbits to immunization with typical Type III pneumococci and a strain related to Type III. The results as a whole show that the relative antigenic effectiveness of these two kinds of related pneumococci, is different in different animals. In rabbits the strain related to but not identical with Type III elicits the better response; in mice, the typical Type III strain is more effective. These relationships furnish an example of the influence of the species of animal upon the effectiveness of even closely related bacterial antigens.

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