objective:to observe the proliferation and differentiation of xenogeneic bone mesenchymal stem cells (bmscs) in the thymus and to explore the feasibility of intrathymic inoculation with xenograft-bmscs in inducing immunotolerance. methods:bmscs of guinea pigs were cultured in medium to p3,and labeled with fetal bovine serum-fe2o3 and 4,6-diamidino-2- phenylindole(dapi).the marked bmscs were inoculated into thymuses gland of rats. the thymuses were sliced up and dyed with perls solution seven days later and observed directly under the fluorescenc microscope. results:bmscs survived long term in the micro-environment of the xenogeneic thymuses,and had the ablitity to differentiate into various cell types. the xenogeneic bmscs had the capablilty of multiple rounds of division. conclusion:it is feasible to induce immune tolerance by intrathymic inoculation with xenograft-bmscs.