Abstract: 【Abstract】 Objective To study the differentiation of human bone marrow mesenchymal stem cells (BMSCs) into Leydig or steroidogenic cells in vivo and the immunoreaction related to transplantation into mouse testis. Methods After differentiation and cultivation, the 3 rd-passage BMSCs were collected and labeled with Hoechest 33342, and joined the saline fluorouracil to form cell-suspending fluid. After injection of the etgane dimethane sulphonate (EDS), the mice received the transplanted cell-suspending fluid by testis net injection with a dose of each side testicular 0.05 mL. Since the first day prior to transplantation, mice were executed every 2 d (one mouse each time) and the testosterone concentrations were analyzed. The control group included 20 BALB/c mice without any treatment during the same period. The results were analyzed by microscopic observation, using 3β-hydroxysteroid dehydrogenase (3β-HSD) monoclonal antibody and mouse anti-human cell nucleus monoclonal antibody for immunofluorescence assay on the testis. For tracking the BMSCs, the cells which were positively stained with both 3β-HSD and mouse anti-human cell nucleus monoclonal antibodies were retrieved. Results Certain killing effect of EDS to the mouse Leydig cells was observed. Transplantation of human BMSCs into the mouse testis by testis net injection was effective and feasible, no immunoreactions were detected. After transplantation, no positive cells of 3β-HSD and mouse anti-human cell nucleus monoclonal antibody were found. Conclusion Transplantation of human BMSCs into the mouse testis by testis net injection was effective and feasible, no immunoreactions were detected. After transplantation, the human BMSCs failed to differentiate into Leydig cells or steroidogenic cells.