OBJECTIVE: To investigate the role of direct and indirect recognition in pig-to-man xenotransplantation. METHODS: Taken the peripheral blood lymphocytes (PBLC) from three Neijiang pigs and two humans as stimulators and respondors, the one-way mixed lymphatic reactions (MLR) of xenograft were carried out, and allo- and self-PBLC as control. RESULTS: Among the three patterns of MLR, syngeneic was MLR the lowest in proliferation, the allogenic MLR was the highest, and the xenogenic MLR was medium. The PBLCs from humans and pigs were matched on HLA-A, B, DR and DQ by means of modified Terasaki assay. The match on pigs was failure because of the pre-existing natural xenogenic antibody in the testing serum. CONCLUSION: The results suggest that the degree of MHC matching still affect the rejection in xenotransplantation, but the present serum assay of MHC matching is not fit for pig.