Complement activation is an essential step in the hyperacute rejection of a vascularized xenograft. Endothelial cell-associated complement regulatory proteins limit complement activation in most settings, but are not able to limit the extensive complement activation that occurs in xenografts, at least in part due to their species specificity. To overcome this problem we and others have sought to express human complement regulatory proteins in the organs of potential donor animals. As an initial step toward evaluating this concept we tested organs from transgenic mice expressing human CD59 and/or decay-accelerating factor (DAF) in two in vitro perfusion systems for the ability to control activation of heterologous complement. In the first system, mouse hearts were perfused on a Lan-gendorff circuit with 50% human plasma. Immuno-pathologic analysis of heart biopsies revealed deposition of human IgG, IgM, and C4 in both control and transgenic organs. The hearts from mice transgenic for human CD59 had substantially less and in some cases no membrane attack complex (MAC) and hearts from CD59/DAF transgenic mice had substantially less or no C5b and MAC. In the second system, mouse hearts were perfused with baboon blood through arterial lines inserted into baboons. Immunopathologic analysis of serial biopsies revealed the deposition of IgG, IgM, and C4 in control and transgenic hearts. Compared with controls, less MAC was deposited in many CD59-expressing hearts and less C5b and MAC in DAF-expressing hearts. These results demonstrate that human complement regulatory proteins expressed in a xenogeneic organ are able to contribute to the control of complement activation in that organ and support the concept that expression of these human molecules would help protect a xenogeneic organ transplanted into a human. © 1995 by Williams and Wilkins.