The reaction of C-reactive protein (CRP) with C-polysaccharide and the choline phosphatides, lecithin and sphingomyelin, previously shown to induce consumption of human complement, failed to effect consumption of guinea pig complement except in the presence of a thermolabile human serum factor. The amount of guinea pig complement consumed by CRP complexes was dependent on optimal proportions of CRP, C-polysaccharide, or phosphatide, and human serum cofactor in the reaction mixture. In subsequent experiments, the serum factor was found associated with human Clq: On DEAE-cellulose chromatography of euglobulin, cofactor activity was associated with the first protein peak and was further enhanced by supplementation with two other chromatographic fractions, consistent with a trimolecular complex of Clq, r, and s. On gradient ultracentrifugation, cofactor activity corresponded to a value of 16.9S in the presence of Ca++ and 12.9S in the presence of EDTA. On filtration through Sephadex G-200 in the presence of Ca++, cofactor and C1 activities were concentrated in the excluded peak. In the presence of EDTA, cofactor activity was also demonstrable in the excluded peak although at lower levels. Fractions with cofactor activity consistently gave precipitin reaction for Clq. Depletion of fresh human serum of Clq by treatment with washed immune precipitate or with aggregated human γ-globulin resulted in complete loss of cofactor activity. Alternatively, addition of purified human Clq provided the cofactor activity for consumption of guinea pig complement. These combined data indicated that the consumption of guinea pig complement by CRP complexes required participation of human Clq, and that these complexes presumably do not interact with guinea pig Clq.