Results are presented to show that protein A (pA) may fix to the Fab region of IgG outside the antigen binding site. Thus, pA-reactive Fab fragments were isolated both from specific anti-egg albumin and specific anti-measles virus antibodies. A probable significance of this Fab reactivity in the precipitation reaction between IgG and pA is discussed. Reactive IgG from human, guinea pig, rabbit, and porcine normal sera all showed similar avidity towards pA. Slight differences between IgG subclasses were, however, observed. The avidity of pA-reactive Fab and Fc fragments, isolated from normal porcine IgG, was highest in the latter.