Summary: Data are given which chart the course of the C′ fixation reaction at 37 C. in experiments with constant amounts of S III and increasing quantities of anti-S III as well as with varying amounts of S III and constant antibody. Analogies and differences between the C′ fixation and precipitin reactions are demonstrated and discussed. A calibrated curve of C′ fixation at constant antibody and varying S III may be used for the estimation of unknown quantities of S III of the order of fractions of a microgram. A calibrated curve of C′ fixation at constant S III and varying antibody may be used for the estimation of microgram quantities of antibody N.