Normal human sera were depleted of C1q, factor D (D) and properdin (P) by a simple and reproducible procedure providing reagents for analysis of complement-dependent functions. Classical pathway activity was restored with purified C1q, and alternative pathway activity with purified D and P. Since both activation pathways were abolished, antibodies and other components could be removed without loss of complement activity during immunoabsorption procedures. Synergism between the two pathways during haemolysis of rabbit erythrocytes was clearly demonstrated, and was also found on analysis of C3 cleavage in serum incubated with other alternative pathway activators such as zymosan and inulin. Experiments with a Neisseria meningitidis serogroup W-135 strain isolated from a patient with inherited P deficiency showed that both pathways were capable of supporting antibody-dependent killing of the bacteria in serum. The alternative pathway was possibly more efficient than the classical pathway in the assay system. In C1q,D,P-depleted serum with high concentrations of anticapsular IgG antibodies, the addition of D alone resulted in efficient alternative pathway-mediated killing. The alternative pathway was equally efficient in a C1q,D,P-depleted serum with low concentrations of anticapsular antibody, but in this case the reaction required both D and P.