The present study introduces an in vitro model of xenogeneic peripheral nerve rejection to analyze the role of macrophages in this complex immunological situation. Nerve-sensitized mouse peritoneal exudate cells were co-cultured with rat peripheral nerve segments. The cultured rat nerve segments were fulminantly rejected in vitro by the co-cultured mouse peritoneal cell population. The massive tissue destruction included Schwann cell damage and was quite distinct from basic myelin phagocytosis observed during Wallerian degeneration in earlier experiments. The nerve-sensitized peritoneal exudate consisted of macrophages and T-cells. Antibody depletion experiments were performed to analyze T-lymphocyte effects in this model. Fulminant tissue rejection depended on the presence of T-lymphocytes in the culture medium. Their presence at the immediate site of tissue rejection, however, was not required. Further experiments were aimed at denning the role of T-cell-derived mediators during in vitro rejection. Depletion experiments using a panel of antibodies to cytokines revealed a critical involvement of IL-2, IL-3, IL-4, IL-6 and interferon-7 in the induction of tissue rejection in vitro. Fulminant tissue rejection in vitro depended on the interaction of these cytokines with macrophages. The participation of macrophage surface receptors was studied in another series of experiments. The macrophage complement receptor type 3 was shown to be critically involved in the phagocytic attack during rejection. Antibodies to MHC class II antigens also abolished fulminant in vitro rejection, indicating that continuous antigen presentation is required in this process.