Human embryonic stem (hES) cells offer an excellent source of cells for transplantation in the treatment of severe diseases. To be clinically safe, the lines have to be derived using strict quality criteria and good manufacturing practice. Animal proteins are immunogenic and may contain microbes, and they should not be used in establishing or propagating hES cells. Derivation systems have been improved towards clinical quality by establishing all 25 hES cell lines using human skin fibroblasts as feeder cells instead of mouse fibroblasts. A further 21 cell lines have been established using synthetic serum instead of fetal calf serum in the medium. In the five latest derivations, the inner cell mass was isolated mechanically instead of by immunosurgery (animal antibodies). Feeder-free derivation would be optimal, but it is not yet considered safe. Clinical-quality lines can be derived by establishing the skin fibroblast feeders in the good manufacturing practice laboratory with human serum in the medium, and by establishing the hES cells on such feeders. In this process, a serum replacement that contains only human protein can be used, the inner cell mass has to be isolated mechanically, and the colonies have to be split mechanically for passaging. Somatic cell nuclear transfer would help to overcome rejection of transplanted cells.