OBJECTIVE To investigate the curative effect and mechanism of allo-bone marrow mes-enchymal stem cell(BM-MSC)infusion in the experimental autoimmune thyroiditis(EAT)mouse model. METHODS An EAT mouse model was established in C57BL/ 6 mice using porcine thyroglobulin(PTg) and Freund adjuvant,while BM-MSCs were injected into the EAT mice of BM-MSC treated group,3×105 per mouse on the 0,7th,14th and 21st day. On the 28th day,all the mice were sacrificed,and thyroid tissue was isolated,embedded in paraffin and stained with HE staining for histological examination. Serum was collected to assess the level of thyroglobulin antibodies( TgAb ) ,thyroid microsomal autoantibodies( TmAb),antithyroid peroxidase antibodies( TPOAb),3,5,3 ',5 '-tetraiodothyronine (TT4),3,5,3 '-triiodothyronine( TT3),interleukin-10( lL-10) and interferon-γ( lFN-γ ). RESULTS① Thyroid tissue in model group showed inflammatory response and infiltration. The level of TmAb, TgAb and TPOAb was significantly increased compared with normal control group,but the level of TT4 was decreased while there was no change in the level of TT3,suggesting that an EAT mouse model was established. ② The thyroid in model group and BM-MSC treated group showed inflammatory response and inflammatory cell infiltration,but the response in BM-MSC treated group was weaker than in model group. ③ Compared with model group,the level of TgAb,TmAb,TPOAb and lFN-γ was decreased obvi-ously(P﹤0.05),the level of TT4 and lL-10 was increased significantly(P﹤0.05),but the level of TT3 changed little in BM-MSC treated group. CONCLUSION BM-MSCs may partly restore the immunologi-cal homeostasis state. The mechanism may be related to its modulation of immune balance of Th1/ Th2.