SummaryThe development of new methodology has resulted in a more profound knowledge of the kinetic and physical properties of the haemopoietic stem cells. Primary and secondary stem cells belong to different cell populations. Only a small fraction of the CFU-S are in cell cycle. Most erythropoietin-sensitive stem cells and about 50% of the CFU-C are synthesizing DNA at any one time. Little is known about the proliferate state of the megakaryocyte precursors. The regulating factors which govern the transi-tion of stem cells from the primary stem cell compartment into one of the secondary stem cell populations are not well known. Different factors are involved : population size control, genetic factors and the inductive micro-environ-ment. Morphological characterisation and identification have shown that primary stem cells are closely related to small lymphoid cells. Further data indicate that the haemopoietic and im-munopoietic stem cells are derived from a common ancester cell.