Abstract： Objective To investigate the activation of complement in the discordant heart xenotransplantation of pig to monkey model. Methods In this experiment,pigs and monkeys were,respectively,selected as donors and recipients,and divided randomly into three groups. In blank group(group A),recipients didn't accept any treatments but heart xenotransplantation. In whole-body irra diation group(group B),recipients received 3 Gy(60Co) whole-body γ-irradiation(WBI) on 28 day before transplantation. In irradiation and intrathymic injection group(group C),the monkeys were pretreated by WBI and the intrathymic injection of pig spleen cells. In every group,monkeys were subjected to heterotopic heart xenotransplantation in order to observe the survival time of cardiac xenografts and the change of C3,CIN6,IgM and IgG. Results The survival time of donor hearts in group C was significantly longer than in group A(P＜0.01). There was a significant reduction of stimulation index in group C as compared with group A in mixed lymphocyte reaction assay. After xenotransplantation, the levels of xenorecative antibody of both IgM and IgG in group C were ascended slower than in group A and group B obviously(P＜0.01). After rejection the levels of CD46 and C3 were declined greatly. Conclusions The pretreatment with intrathymic injection and WBI could induce T cell immune suppression,whereby to restrain production of elicited xenorecative antibodies of both IgM and IgG classes. However,it could not hinder activation of complement during the hyperacute rejection and consequent xenograft rejection.