Differences and/or similarities of the accessory cell activity of circulating peripheral blood monocytes (PBM) and intestinal lamina propria macrophages (LPM) in the pig have never been evaluated. Therefore this study was designed to compare the accessory cell activity of these cells in the induction of primary allogeneic mixed leukocyte responses (MLR) and polyclonal mitogenesis. Splenic adherent cells (SPAC) were used as control accessory cells. It was observed that PBM, LPM and SPAC induced significant T cell proliferation in response to allo-antigens. Significantly higher responses were elicited by SPAC, followed by PBM whereas, LPM induced low responses when cultures were established a T cell: accessory cell ratio of 1:1. PBM was the only accessory cell which induced significant responses at a cell ratio of 10:1. Whereas LPM and SPAC were able to induce significant oxidative mitogenesis, PBM were poor stimulators of this response. The observed difference between PBM, SPAC and LPM is attributed partly to differences in number of major histocompatibility complex (MHC) class II positive cells and the intensity of expression of the glycoproteins.