Investigated was the effect of various amino agents (stained and phthalic anhydride and sulfopicric acid) on the complement-and antigen-fixing activity of IgG and IgM antibodies isolated from guinea pig sera, the donor animals being hyperimmunzed with type O, strain Polyana, of the foot-and-mouth disease virus, At the moderate modification (75-86 per cent acetylation, and 36-40 per cent 'phthalation') of the primary amino groups the foot-and-mouth disease antibodies retained only half of the conjugation caused greater and complete loss of this activity. IgG and IgM showed almost equal specificity with regard to the antigen that had induced them. The lowered complement-fixing capacity was shown to have no specific effect on the primary relation of the foot-and-mouth disease virus to the antibody. Chemical modifying agents inactivated the specific small, strongly defined complement-fixing zones in the Fc-fragment of the foot-and-mouth disease antibody molecule.