1. Defects in methods previously proposed for the estimation of complement components are: failure to ensure an excess of the desired components and failure to ensure absence of anticomplementary effects in the dilution ranges used. Existing data are therefore subject to these uncertainties.
2. Methods are proposed for controlling the adequacy of the reagent for each component, for using it in dilutions below its anticomplementary range, and for reinforcing it with necessary components if these are present in inadequate amounts.
3. Titrations are given of the four components in human and guinea pig complements and in "midpiece," "endpiece," and in the various reagents used, including one with C'3 reactivity, solely, and one withC'4 reactivity.
4. C'3 is shown to be the component which usually limits the titer in guinea pig complement, and C'2 the component of lowest titer in human complement.
5. In immune hemolysis, each component of human complement may be replaced by the corresponding component of guinea pig complement and vice versa.