Assay systems for human haemopoietic stem cells are necessary for understanding the regulation of haemopoietic cell production. The stem cells are ultimately responsible for the production of mature blood cells but constitute only a tiny minority of the total haemopoietic cell population. The remainder is a heterogeneous mixture of cells at various stages of differentiation and maturation along the myeloid, erythroid and megakaryocytic lineages. Although clonogenic assays for the lineage-committed progeny of human stem cells are well established and have produced a wealth of information about the regulation of the later stages of haemopoiesis, there is no universally accepted assay for self-renewing pluripotent haemopoietic stem cells in human haemopoietic tissue. Nevertheless, several types of in vitro and in vivo assay systems have been devised for very early haemopoietic cells in man. These can be broadly divided into three classes: (1) direct clonogenic assays; (2) assays where non-clonogenic cells are detected by their ability to produce more mature clonogenic progeny and (3) transplantation of human haemopoietic cells into immunocompromised (SCID) mice. The different assay systems are designed to answer important questions about the control of haemopoietic stem cell proliferation and differentiation and to provide qualitative and quantitative information about stem cell numbers and identity.