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, Mg
2+ and C' was investigated. It was found that this phenomenon was closely related to the flexible conformation of DNA which is highly influenced by Mg
2+. Further, this effect could be eliminated in the presence of 5mM MgCl
2. The mechanism of the elimination by Mg
2+ of the anticomplementary effect of DNA was discussed.
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