Factor B activity of the properdin system was found to be identical with purified glycine-rich beta-glycoprotein (GBG) but was distinct from the normal human serum protein capable of forming a C3-inactivating complex with a protein from cobra venom (CoF). Factor B activity coincided with electrophoretically separated GBG genetic variants, whereas the CoF-binding protein did not. GBGase destroyed factor B as it cleaved GBG but did not destroy the C3-inactivating activity of the CoF-binding protein. During incubation of serum with CoF, GBG did not change in molecular size, nor was there any coincidence in the immunoelectrophoretic mobilities of CoF and GBG. It was not possible to precipitate labeled CoF incubated with serum by anti-GBG, nor labeled GBG from serum incubated with CoF by anti-CoF. The CoF-binding capacity of serum was 2 mg/100 ml or less or under 6.5% of the serum concentration of GBG. When labeled CoF was added to serum below the binding capacity, complete complexation of the CoF was demonstrated, whereas CoF was largely uncomplexed when CoF was added in amounts equimolar to GBG.