Summary: The serum of a subject vaccinated with diphtheria toxoid and two sera from patients recovering from infections with Brucella abortus suis were fractionated and samples consisting of γss- (7 S γ), γ1M- (19 S γ) and γ1A- (β2A-) globulins were separately assessed for antibody activity. All three protein species appeared to be serologically active. Particular care was taken with the interpretation of data concerning the γ1A-globulin samples. The serologic activities of the γ1A samples could not be explained on the basis of any contamination with γss-globulin, and were ascribed to γ1A-globulin itself. A peculiar feature of the γ1A anti-Brucella agglutinins here studied was their absence of complement-fixing capacity. It is suggested that future investigations on the physicochemical nature of antibodies be carried out with the knowledge that γ1A- (β2A-) globulin constitutes a third class of antibodies, besides the γss- (7 S γ) and γ1M- (19 S γ) classes.