Background: It has been speculated that host macrophages contribute to rapid clearance of transplanted xenogeneic cells. To address such a possibility, phagocytotic and cytolytic activities of human macrophages toward xenogeneic porcine cells were evaluated in vitro in the absence of antibodies and complement factors. Methods: Human peripheral monocyte-derived macrophages (P-macrophages) and reticulo-endothelial macrophages (RE-macrophages) were obtained from volunteers' peripheral blood and from the perfusion effluents of liver allografts for transplantation, respectively. 5-(and 6-) carboxyfluorescein diacetate succinimidyl ester (CFSE)-labeled human autologous red blood cells (auto-RBCs), ABO-incompatible RBCs (incom-RBCs) and xenogeneic porcine RBCs (xeno-RBCs) were incubated with the human macrophages; subsequently, the macrophages that had phagocytosed the RBCs could be identified as CFSE positive cells by FCM analyses and confocal microscopy. Cytolytic activity was quantified by calculating levels of lactate dehydrogenase in each culture supernatant. Results: Human RE-macrophages spontaneously phagocytosed and had a remarkable cytolytic activity toward xeno-RBCs, but not toward auto-RBCs or incom-RBCs. Elimination of α-galactosyl xenoantigen (α-Gal) epitopes on xeno-RBCs did not prevent phagocytotic or cytolytic activity of RE-macrophages. Conclusions: These findings indicate phagocytotic and cytolitic activities of human macrophages toward porcine cells are initiated by a factor other than α-Gal in a mechanism independent of antibody/complement opsonization. © Blackwell Munksgaard, 2005.