Human CD46 (membrane cofactor protein) is a type 1 glycoprotein that functions to protect autologous cells from complement-mediated damage by binding C3b and C4b for their factor I-mediated cleavage. We now describe the production and function of recombinant soluble CD46 (rsCD46), which was produced as a truncated form by mutagenesis using the splice overlap extension polymerase chain reaction, by inserting a translational stop codon into the CD46 cDNA at the junction of the transmembrane and extracellular domains. After transfection of an expression construct into 293-EBNA (Epstein-Barr nuclear antigen)-transformed cells, secretion of rsCD46 protein was detected by immunoradiometric assay using monoclonal antibodies. Following a single-step immunoaffinity purification, the protein resolved as a single band of approximately 56,000 MW on sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). The purified rsCD46 (51 micrograms/ml) protected Chinese hamster ovary (CHO) cells from lysis initiated by a high titre rabbit anti-CHO antibody and complement from rabbit or human. The protection was specifically mediated by rsCD46 because the monoclonal antibody M177, which blocks interaction between CD46 and C3b/C4b, abrogated the protection. The results demonstrate that rsCD46 is effective as a fluid-phase regulator of complement activation on cell surfaces, even when initiated by the classical complement pathway. The in vivo efficacy of rsCD46 was investigated using a mouse heart to rat xenograft model. Administration of a bolus injection of rsCD46 was effective at delaying hyperacute graft rejection. These data suggest that rsCD46 may have a role as a therapeutic agent.