Cobra venom factor (CVF) is the complement-activating protein in cobra venom. It is a three-chain glycoprotein with a molecular weight of 149,000 Da. In serum, CVF forms a bimolecular enzyme with the Bb subunit of factor B. The enzyme cleaves C3 and C5, causing complement consumption in human and mammalian serum. CVF is frequently used to decomplement serum to investigate the biological functions of complement and serves as a tool to investigate the multifunctionality of C3. Furthermore, CVF bears the potential for clinical application to deplete complement in situations where complement activation is involved in the pathogenesis of disease. CVF was isolated from Indian cobra (Naja naja naja) venom. The protein was crystallized at room temperature using the sitting-drop vapour-diffusion technique. The crystals diffract to 2.7 A resolution and belong to the tetragonal space group P4(1), with unit-cell parameters a = b = 62.7, c = 368.1 A.