Rabbit α-globulin, β1-globulin and subfractions of γ-globulin were aggregated by coupling with bis-diazotized benzidine and the aggregated products were studies for skin reactivity in normal guinea pigs and for C′-fixing properties. Induction of these biological activities upon nonspecific aggregation was characteristic of γ-globulin only. Aggregated products of neither α-globulin nor β1-globulin had either activity. Evidence was presented that only γ-globulin had affinity for guinea pig tissues to which antibody combines upon passive sensitization. All three of these properties, namely skin reactivity and C′-fixing activity upon aggregation, and affinity for guinea pig tissue were present approximately equally in all the chromatographic subfractions of γ-globulin. All γ-globulin subfractions of rabbit anti-ovalbumin sera contained antibody. Soluble antigen-antibody complexes prepared with these fractions gave skin reactions in normal guinea pigs and inactivated C′. On a weight basis, the soluble complexes and nonspecifically aggregated γ-globulin were comparable with respect to these biologic activities.