Murine embryonic stem (ES) cells are permanent blastocyst-derived cell lines capable of contributing to a wide variety of tissues, including the germ line, after injection into host blastocysts. Recently, we have shown that ES cells can produce all of the cells of the developing fetus after aggregation with developmentally compromised tetraploid embryos. Completely ES cell-derived embryos die perinatally, but the liver of these embryos is a source of entirely ES cell-derived hematopoietic progenitors. We have taken 14- to 15-day fetal liver cells from ES cell-tetraploid chimeras and reconstituted the hematopoietic system of lethally irradiated adult recipient mice. ES cell-derived hematopoietic stem cells were capable of long-term (greater than 6 months) repopulation of irradiated recipients, and all hematopoietic cell lineages analyzed (erythrocytes, T cells, mast cells, and macrophages) were derived exclusively from ES cells in such recipients. Thus, ES cells retain the capacity to differentiate into all hematopoietic cell types after prolonged passage in culture. This approach should provide a direct route to the production of mice whose hematopoietic cells carry genetic alterations that would be lethal if passed through the germ line.