Aggregated human γ-globulin, obtained by heating, inactivated C′1 and/or C′4, as well as C′2 and C′3. With respect to the successive steps of the complement inactivation, the aggregated γ-globulin fixed C′1,4, followed by inactivation of C′2. Inactivation of C′3 in whole complement by the aggregated γ-globulin was inhibited in the presence of EDTA, due to blocking of the preceding steps. The similarity in the mechanism of complement fixation by antigen-antibody systems and by aggregated γ-globulin was discussed.