Background.It is now generally accepted that complement activation is critical for the hyperacute rejection of xenografts. Activation of the classical pathway as the result of the interaction of xenoreactive IgM xenoantibodies with the vascular endothelium has been observed in all species combinations examined to date. A number of studies using a variety of species combinations have also implicated alternate pathway involvement; however, these studies do not enable a conclusion to be drawn as to whether the alternate pathway can be activated in the complete absence of classical pathway activation.Methods.In this study, human plasma was depleted of both C1q and factor D and then reconstituted with purified C1q or factor D to restore the classical and alternate complement pathways, respectively. The ability of these modified plasmas to prosecute hyperacute rejection was then examined using an ex vivo isolated mouse heart perfusion model based on the Langendorff system.Results and Conclusions.In the mouse to human species combination, both the classical and alternate pathways of complement are independently capable of initiating complement activation and mediating xenograft rejection.