Complement receptor type 1 (CR1, CD35) contains both factor I cofactor activity and convertase decay accelerating activity, but is, in general, thought to be an extrinsic regulator of complement activation. In this study, we prepared a phosphatidylinositol (PI)-anchored mini-CR1, which is composed of the short consensus repeat (SCR) 8-11 of CR1 and the PI anchor of DAF, and expressed it stably on a swine endothelial cell (SEC) line. We then examined the intrinsic regulatory activity of the mini-CR1, with respect to complement-mediated cell lysis as an in vitro hyperacute rejection model of a swine to human discordant xenograft. Flowcytometric profiles of the stable SEC lines with mini-CR1 showed a moderate level of expression for this molecule. Mini-CR1 blocked human complement-mediated cell lysis by approximately 50-70% on SEC. From the data of this study and our previous studies, mini-CR1 was more effective than membrane cofactor protein (MCP, CD46), and as effective as decay accelerating factor (DAF, CD55) in this system. The results suggest that PI-anchored mini-CR1 represents a useful molecule for clinical xenotransplantation.