Objective To establish C57BL/6J embryonic stem (ES) cell lines with potential germline contribution Methods ES cells were isolated from blastocyst inner cell mass of C57BL/6J mice, and cultured for 15 passages, and then injected into blastococels of lCR mice blastocysts to establish chimeric mice.Results Three ES cell lines (mC57ESl,mC57ES3, mC57ES7) derived from the inner cell mass of C57BL/6J mice blastocysts were established. They were characteristic of undifferentiated state, including normal XY karyotype, expression of a specific cell surface marker "stage-specific embryonic antigen-1" and alkaline phosphatase in continuous passage. When injected into immunodeficient mice, mC5 7ES1 cells consis tently differentiated into derivatives of all three embryonic germ layers. When mC57ES1cells were transferred into ICR mice blastocysts, 4 chimeric mice have been obtained.One male of them revealed successful germ-line transmission. Conclussion We have obtained C57BL/6J ES cell lines with a potential germ-line contribution, which can be used to generate transgenic and gene knock-out mice.