Complement plays a major role in hyperacute rejection of xenografts. In order to overcome this, we are developing, by minimal mutagenesis, a modified C3 molecule that, like cobra venom factor (CVF), escapes normal complement regulatory processes and inhibits complement-mediated responses by systemic depletion of C3. Unlike CVF, this protein should have little or no immunogenicity and be suitable for repeat administrations. As an initial step in this process, we have modified human C3 to make it resistant to inactivation by factor I. The factor I resistant C3 is capable of forming an active C3 convertase. Preincubation with normal human serum abrogated subsequent complement-mediated cytolysis by both the classical and alternative pathways, while wild-type (wt) C3 was inactive. The modified human C3 also blocked complement activity of guinea-pig serum. For economical and rapid production, we have developed expression of recombinant C3 wt and mutant proteins in the Baculovirus system. Large quantities are also being produced from stably transfected CHO cell lines. In addition, we have developed a fast C3 purification method by engineering a 6XHIS tag into the C3a portion of the molecule, thereby avoiding the need for subsequent separation of the tag from active C3b molecules.