This chapter describes the development of clonal stromal cell lines through transforming bone marrow stroma with the SV40 large tumor (large T) antigen oncogene. The chapter also discusses the subsequent manipulation of expression of SV40 large T with a synthetic metallothionein promoter, allowing the controlled proliferation of the stromal cell lines. The successful support of highly purified hematopoietic progenitor cells obtained from cord blood by these cell lines is discussed. Such a reductionist approach has allowed generating a system, which provides a greatly simplified model for the analysis of the complexities of hemopoietic regulation at the molecular level. CD34-positive progenitor cells were purified to near homogeneity from cord blood by an immunorosetting technique followed by cell sorting. The CD34 + Lin- cells were cocultured on MT4-SV40 stromal cell lines. These cloned lines were shown to be able to support proliferation and maintenance of immature colony forming cells in culture for at least 7 weeks.