When a constant quantity of denatured DNA required for C-fixation reaction was employed as the antigen, an anticomplementary action of DNA was frequently observed. In order to cope with this difficulty in the analysis of C-fixation, the relationship between DNA, Mg2+ and C was investigated. It was found that this phenomenon was closely related to the flexible conformation of DNA which is highly influencd by Mg2+. Further, this effect could be eliminated in the presence of 5MM MgCl2. The mechanism of the elimination by Mg2+ of the anticomplementary effect of DNA was discussed.