Summary: The minimum size of nonspecifically aggregated rabbit γ-globulin having skin reactivity and C′-fixing properties was studied. The results indicated that 9.2 to 9.6 S component in normal and biphenylene azo-RGG preparations had both activities, whereas nonaggregated γ-globulin of 6.6 S did not have either activity. The C′-fixing activity of the 9.6 S component was less than that associated with aggregates larger than this component. The smallest aggregate of piece III of the γ-globulin having both activities was comparable to or slightly smaller than 7 S γ-globulin. The results suggested that induction of these biologic activities was associated with dimerization of γ-globulin or piece III.