Embryonic stem (ES) cells are totipotent cells derived from cultured preimplantation blastocysts. When injected into embryos, they can give rise to all somatic lineages as well as functional gametes. Embryonal carcinoma (EC) cells are pluripotent cells, derived from teratocarcinomas, which contribute to somatic lineages, but only rarely to the germ line. A novel source of pluripotent cells, remarkably similar to both ES and EC cells, has been identified. These are EG cells, derived by culturing primordial germ cells isolated from postimplantation embryos, but it is not known whether they resemble ES or EC cells in their ability to contribute to the germ line. Here we show that EG cells of both sexes can form functional gametes. The derivation of such cell lines offers a new route to deriving totipotent cells and also provides insights into the timing of X-chromosome inactivation/activation and genomic imprinting in the germ line.