A new embryonic stem (ES) cell line, H3, has been established from a 3.5-day p.c. blastocyst of the 129/sv strain of mice. The degree of germ-line competence of this cell line was compared with that of J1 sublines in the FvB/N host background, an albino inbred strain which is preferably used in the generation of transgenic mice via pronucleus injection. All chimeras of the FvB/N background derived from H3 or J1 cells showed a low degree coat color chimerism. However, contrary to the previous report on the germ-line incompatibity of the FvB/N strain with 129-derived ES cells, H3 cells gave germ line transmissions in five out of six FvB/N based chimeras. This property of H3 cells contrasted with those of J1 cells which did not give any significant germ line transmission from chimeras made in the same FvB/N host whereas they were efficient in forming a germ-line chimerism in the C 57BL/6J background. These results conform to the previous observations by others that the germ-line compatibility of ES cells with a host strain is dependent on clonal lines. FvB/N mice are known to be a robust breeder and the females respond well to hormone-induced superovulation yielding a large number of germ-line compatible blastocysts. Such advantages of FvB/N may be utilized in the study of gene targeting when this host is combined with H3 ES cells, and will results in saving costs and efforts required for such experiments.