Abstract: Objective: To study on differentiation of marrow-derived endothelial progenitor cells(EPCs) in vivo,and to further reveal EPCs' biological characteristics.Methods: Rat bone marrow-derived EPCs were separated by density gradient centrifugation,and cultrued in vitro;and then labelled with Hoechst 33342 and collected.The labelled EPCs were injected subcutaneously into nude mice's back,on the both sides of the spinal.Animals were observed daily and measured the size of the subcutaneous masses.At the 50th day after transplantation,the subcutaneous masses were removed and then performed HE and immunofluorescent staining to detect the expression of antigens CD31,Flk1,CD34.Results: At the 20th day after transplantation,we observed the subcutaneous masses with size of(0.13±0.018) cm² on the nude mice's back,and then the subcutaneous masses were gradually increasing.At the 40th day,EPCs began to get into period of growth stagnation.Tube-like structure,muscle tissue and fatty-like cells were observed through HE staining,at the same vision,blue fluorescent-marked cells were observed through fluorescence microscopy.The result of Immunofluorescence staining showed that some of the blue fluorescent cells expressed antigens CD31,Flk1,CD34.Conclusions: The part of the EPCs transplanted to nude mice differentiated into endothelial-like cells and formed tube-like structure,but the other part differentiated into non-endothelial cells.