ABSTRACTTissue culture assays of human hematopoietic progenitor cells have advanced the study of the physiology of hematopoiesis at its early stages and have yielded knowledge of some of the factors that regulate cell differentiation and replication. Our understanding of the pathophysiology of various forms of bone marrow failure has also increased and will provide a more rational basis for treatment. Since immune mechanisms of marrow failure can be distinguished from intrinsic hematopoietic stem cell defectsin vitro.the culture assays should allow an appropriate selection of disorders and individuals to be treated by bone marrow transplantation.