Summary: A simplified and improved quantitative method for the estimation of the fixation of C′ is described. A single value of 1/n, 0.2, suffices for many variations in experimental conditions. Adsorption of protein antigens by glass at the high dilutions used may be avoided by dilution with 0.1 per cent inactivated serum. Data are given leading to the choice of 90 min as the most suitable fixation period at 37 C. Comparisons are presented of the extent of C′ fixation at 37 C and ice-box temperatures in the presence of varying amounts of C′.