Summary: With several antigen-antibody systems, C′ fixation reactivity of whole serum and of separated 19 S and 7 S antibody components was examined. Antigens which did fix C′ with appropriate 19 S antibodies included: bacterial lipopolysaccharides, cardiolipin, denatured DNA and adenosine conjugated to poly-L-lysine or to lysinerich histone. No C′ fixation was observed with 19 S antibodies and hemocyanin or with adenosine-human serum albumin or adenosine-ribonuclease conjugates. Corresponding 7 S antibodies fixed C′ with all of the above antigens. All 7 S antibodies reacted much more effectively at 4°C than at 37°C. In the systems in which 19 S antibodies did fix C′, the ratio of reactivity at 4°C to that at 37°C was much lower than was the case with 7 S antibodies, with both human and rabbit antisera. In the reaction of anti-adenosine macroglobulins with adenosine conjugated to lysine-rich histones, an increase in the average number of adenosine molecules substituted on the histone was accompanied by a fall in the ratio of 4°C effectiveness to 37°C effectiveness. C′ fixation with macroglobulin antibodies may require both an antigen of relatively large molecular volume and critically spaced repeating determinants on an antigen molecule.