Two cobra-venom factors, one from Naja n. naja (CVFn), the other from Naja h. haje venom (CVFh), have been purified and compared, functionally and structurally. Both factors interacted with human factors B and DS to form a potent C3 convertase, CVFBb. However, while the convertase formed with CVFn did also efficiently cleave C5, CVFhBb had very little C5-cleaving potency only, in particular when human C5 was used as substrate. Studies with agarose-linked CVF preparations indicated that CVFh has only low binding affinity for C5gp and C5hu whereas CVFn binds to both C5 species with much higher affinity. Since C5-binding (to CVF or to C3b) is a prerequisite for its cleavage by C3/C5 convertases, the difference in binding potency explains the different C5-cleaving activity of the two CVF preparations. When a ligand for C5, surface-fixed C3b, is present, CVFhBb is also capable of cleaving C5. The difference in activities of CVFn and CVFh is reflected in their different potency to interfere with immune haemolysis and in causing indirect lysis by their complexes with activated factor B. By gel chromatography of the CVF preparations in C5-containing medium, a stoichiometric complex CVFn-C5 (1 + 1) could be demonstrated. An analogous complex of C5 was neither found with CVFh, nor with C3hu or soluble C3bhu. Structural differences between CVFn and CVFh were revealed by immunodiffusion analysis and by polyacrylamide-gel electrophoresis in presence of SDS. The data available so far provide, however, no clear information about the structure of the C5 binding site.