1. Antisera prepared by immunizing rabbits with C. diphtheriae gravis, mitis, and intermedius and C. hofmannii contain antibodies which will give complement-fixation reactions with alcoholic extracts of the organisms.

2. By means of cross-absorption experiments three different lipoid antigens can be demonstrated: (a) a specific antigen h present only in C. hofmannii; (b) a specific antigen d characteristic of C. diphtheriae mitis but probably present also in small amount in C. diphtheriae gravis and intermedius; and (c) a non-specific or group antigen G present in large amount in C. diphtheriae gravis and intermedius and C. hofmannii, and in small amount in C. diphtheriae mitis; this G antigen is probably made up of a number of components.

3. Sera of human diphtheria convalescents may contain antibodies reacting with alcoholic extracts of C. diphtheriae. Sera from mitis cases usually react only with extracts of G. diphtheriae mitis, i.e. they contain only d antibody, while sera from gravis or intermedius cases usually contain only G antibody and react with extracts of C. diphtheriae gravis, intermedius and C. hofmannii, but not with extracts of G. diphtheriae mitis.