Summary: A method for the titration of human complement is described. The existence of at least four components in human complement, similar to, though not entirely identical with the components of guinea-pig complement, is established. Methods are given for the preparation of human complements lacking respectively in C′1, C′2, C′3 and C′4, and for the reactivation of these. A method is given for the separation of human complement into two individually inactive fractions which are fully active upon recombination. This separation is achieved by dialysis of human serum against a phosphate buffer of ionic strength 0.02 and pH = 5.4. The two fractions obtained correspond functionally to the end-piece and mid-piece of guinea-pig complement. It was found that C′3 is the only component which is effectively mutually substitutive in human and guinea-pig complement systems. C′1 of cow and sheep sera were found effective in replacing human C′1. The fortification of human complement by both guinea-pig and human specifically inactivated complements was attempted. Human “pH = 5.4 − μ 0.02 supernate” was found effective in fortification of human complement; guinea-pig C′3, and to a lesser degree C′2, were found to be involved in the fortification of human complement by guinea-pig specifically inactivated complements.