An in vivo model system to understand the mechanism of xenograft rejection was established using human peripheral blood leukocyte-reconstituted SCID (hu-PBL-SCID) mice. Human xenoreactive natural antibodies (XNA), of IgM and IgG subtypes, capable of binding to pig aortic endothelial cells (PAEC) were detected in the sera of hu-PBL-SCID by ELISA and flowcytometric methods. Western blot analysis of PAEC lysates showed that IgM and IgG XNA from hu-PBL-SCID recognized xenoantigens with similar molecular mass as those recognized by XNA from normal human serum (NHS). This result demonstrated that hu-PBL-SCID contained XNA representing the same repertoire as that of the NHS. XNA from NHS and hu-PBL-SCID were also able to induce intracellular Ca2+ signals in cultured PAEC several fold above the basal level. This result revealed their functional similarity and demonstrated for the first time that XNA in the absence of C can activate PAEC, which may lead to the pathology of xenograft rejection. In vivo, PAEC transplanted under the kidney capsule of hu-PBL-SCID mice showed deposition of human IgM and mouse C. In summary, the present study demonstrates that hu-PBL-SCID can serve as a useful model to characterize innate immunity against xenograft.