EBs from mouse pluripotent stem cells grown on feeder layers or in suspension may contain a variety of cell types. These aggregates range from a cluster of pluripotent stem cells enclosed by a layer of endoderm to complex structures closely resembling an embryo during early development. Embryoid bodies (EBs) are differentiated cell aggregates first described as arising in human ( 17) and mouse ( 18– 20) teratomas and teratocarcinomas. The pluripotency of ES and EG cells can be demonstrated in vitro and in vivo. Pluripotent stem cell lines that share most of these characteristics also have been reported for chicken ( 10), mink ( 11), hamster ( 12), pig ( 13, 14), rhesus monkey ( 15), and common marmoset ( 16). Other important characteristics include growth as multicellular colonies, normal and stable karyotypes, the ability to be continuously passaged, and the capability to differentiate into cells derived from all three embryonic germ layers. These properties are characteristic of, but not specific for, pluripotent stem cells. Mouse ES and EG cells share several morphological characteristics such as high levels of intracellular alkaline phosphatase (AP), and presentation of specific cell surface glycolipids ( 7, 8) and glycoproteins ( 9). Both ES and EG cells are pluripotent and demonstrate germ-line transmission in experimentally produced chimeras ( 5, 6). Embryonic stem (ES) cells are derived from the inner cell mass of preimplantation embryos ( 1, 2), and embryonic germ (EG) cells are derived from primordial germ cells (PGCs) ( 3, 4). Pluripotent stem cells have been derived from two embryonic sources. Based on their origin and demonstrated properties, these human PGC-derived cultures meet the criteria for pluripotent stem cells and most closely resemble EG cells. Immunohistochemical analysis of embryoid bodies collected from these cultures revealed a wide variety of differentiated cell types, including derivatives of all three embryonic germ layers. Both XX and XY cell cultures have been obtained. The cultured cells have been continuously passaged and found to be karyotypically normal and stable. Throughout the culture period most cells within the colonies continued to be alkaline phosphatase-positive and tested positive against a panel of five immunological markers (SSEA-1, SSEA-3, SSEA-4, TRA-1–60, and TRA-1–81) that have been used routinely to characterize embryonic stem and EG cells. Over a period of 7–21 days, PGCs gave rise to large multicellular colonies resembling those of mouse pluripotent stem cells termed embryonic stem and embryonic germ (EG) cells. Initially, single PGCs in culture were visualized by alkaline phosphatase activity staining. With the goal of establishing pluripotent stem cell lines, gonadal ridges and mesenteries containing primordial germ cells (PGCs, 5–9 weeks postfertilization) were cultured on mouse STO fibroblast feeder layers in the presence of human recombinant leukemia inhibitory factor, human recombinant basic fibroblast growth factor, and forskolin. Human pluripotent stem cells would be invaluable for in vitro studies of aspects of human embryogenesis.