Control of the formation of complement C5-convertase is an important factor in maintaining human homeostasis. Some physiologically active endogenous and exogenous substances were shown to be capable of exerting such control. A method was developed for studying the influence of the effectors on the formation of classical-complement-pathway C5-convertase from C3-convertase stabilized by nickel ions. Immunoglobulins G and M, recombinant alpha and gamma interferons, serotonin, epinephrine, histamine, and ethanol were studied as effectors. The binding constants of the effectors with C3b, the active C3 component, and the values of the maximal effect were determined to evaluate the physiological importance of the effectors. The inhibition of C5-convertase generation was observed in all cases except for IgG and IgM (rheumatoid factor), which showed no effect and activating effect, respectively.