Summary: The formation of anticomplementary aggregates which occur when 7 S γ-globulin is heated is suppressed by the presence of albumin or of α- or β-globulins. However, these non-γ-globulin serum protein fractions will not prevent the further aggregation of 9.5 S to 1000 S γ-globulin materials to precipitated components devoid of anticomplementary activity. Hence anticomple-mentary activity cannot be induced in whole serum by heating, and if present in normal “convalescent” whole serum can be abolished by heating the serum at 60 to 65°C. Experiments utilizing heated and nonheated preparations of purified macro-γ-globulin (19 S) and purified nonaggregated 9.5 S γ1A-globulin indicated that molecular size per se is not responsible for the anticomplementary activity of aggregates and further suggested that such activity is restricted to the 7 S component of the three immunoglobulins.