We have investigated the kinetics of colony formation by progenitor cells in chronic myeloid leukaemia (CML) using erythroid burst-forming units (BFU-E) as a model system. For this, we scored the numbers of subcolonies produced by individual BFU-E in cultures of normal marrow and blood cells and in cultures of CML blood cells. The formation of an erythroid burst consisting of a single subcolony was taken as evidence for immediate terminal differentiation; the formation of multiple subcolonies was taken as evidence for commitment to terminal differentiation only after several cell generations. Therefore the probability of differentiation can be obtained by scoring the numbers of subcolonies in individual erythroid bursts. We found that the probability of differentiation is decreased (P = 0.0004) and the number of subcolonies increased (P = 0.01) in CML BFU-E compared with normal BFU-E. The cellularity of the BFU-E was also increased in CML. Using the probabilities of differentiation and renewal obtained from the BFU-E cultures the results fitted the predictions of a stochastic branching model. These results indicate that (a) commitment to terminal erythroid differentiation occurs over several cell generations in populations of BFU-E, (b) the probability of commitment to terminal differentiation (PD) within a particular population of BFU-E, remains a constant independent of the number of cell generations involved, (c) PD is lower during burst formation by CML BFU-E than by normal BFU-E, and (d) commitment to terminal differentiation occurs over more cell generations in CML burst formation than in normal burst formation. Therefore a reduced probability of differentiation may be a primary defect and could explain the expansion of the erythroid progenitor cell compartment in CML.