The application of somatic cell gene therapy to large patient populations will require the development of safe and practical approaches to the generation and characterization of genetically manipulated cells. Transkaryotic implantation is a gene therapy system based on the production of clonal strains of engineered primary and secondary cells, using non-viral methods. We demonstrate here that, on implantation, these clonal cell strains stably and reproducibly deliver pharmacologic quantities of protein for the lifetime of the experimental animals.