An anti-complementary protein which cross-reacts with antiserum to cobra venom factor (CoF) has been highly purified from Crotalus atrox venom (CAV). On an equal weight basis, the C. atrox factor (CA-F) had one-third the anti-complementary activity of CoF and similarly consumed the terminal components of complement via the alternative pathway. The subunit compositions of CoF and CAV were similar, each being composed of alpha and beta chains held together by covalent and non-covalent bonds. Agarose gel diffusion analysis using monospecific antiserum to CoF detected a reaction of partial immunological identity between CoF and CA-F and between C3 and CA-F with the CoF and C3 precipitin lines spurring over the CA-F lines. Thus CA-F probably represents a C. atrox C3 protein which is capable of activating the alternative complement pathway via the amplification loop in a manner analogous to that of C3b and CoF.