According to current dogma, circulating blood cells are all derived from the same progenitor, which therefore must be both pluripotent and capable of prolific self-replication. In the irradiated mouse, such haemopoietic stem cells (HSC) give rise to splenic colonies, and thus are designated as CFU-S (colony forming units-spleen). Definitive identification of a similar entity in man so far has proved elusive. However, primitive unipotent cells, committed to development along a single pathway, can be detected in human blood-forming tissues under appropriate culture conditions. Operationally defined as CFU-E (erythrocyte), CFU-GM (granulocyte/macrophage) etc., the ontogenetic relationships of these cells to each other and to the HSC have been the objects of exhaustive study. A population of lymphocytes, classed as ‘null’ cells, do not exhibit the surface membrane markers which characterize commitment to differentiation in the thymusdependent (T-cell) or bursa- equivalent (B cell) lineages. Accumulating evidence points to the null lymphocyte as a potential precursor of haemopoietic tissue. In bone marrow, the activity of terminal deoxynucleotidyl transferase (TdT) is concentrated in null cells, but the relevance of this unique enzyme to lymphocytic differentiation remains uncertain. Nevertheless, it appears that the functional heterogeneity, expressed within the family of lymphocytes, extends to haemopoiesis. The roles of various lymphocyte populations, in the generation and functional control of blood-forming tissue, are examined in this review.