Six rabbit antisera induced by complexes of denatured deoxyribonucleic acid (DNA) and methylated bovine serum albumin (MBSA) were found to have C′ fixation properties which differed from those of human (lupus erythematosus serum) anti-DNA antibodies. The rabbit anti-DNA-MBSA antibodies: 1) fixed C′, with denatured DNA, more efficiently at 37°C than at 4°C; 2) were eluted from Sephadex G-200 columns in Fraction I as macroglobulins; and 3) were virtually completely inactivated by treatment with 0.1 M mercaptoethanol without subsequent alkylation. In these respects, lumpus erythematosus antibodies, in C′ fixation reactions with DNA, behaved similarly to an anti-protein (anti-ribonuclease) system. They reacted more efficiently at 4°C than at 37°C; they were eluted from Sephadex G-200 columns in Fraction II as 7S globulins; and their C′ fixation activity was only moderately reduced by 0.1 M mercaptoethanol. The lupus erythematosus sera also showed greater variability than the rabbit sera in the degree to which they reacted with native and denatured DNA.