Alterations in neoplastic cell behaviour responsible for increased production of terminally-differentiated granulocytes during long-term culture of bone marrow in different categories of acute leukaemia and myelodysplasia have been investigated. An increase in neutrophils associated with transition to a morphological picture identical to normal control cultures occurred in 15 of 25 studies on acute leukaemia in contrast to one of six studies on myelodysplastic disorders. An abnormal neoplastic karyotype was employed as a marker for monitoring the course of the neoplastic cell population in 11 studies in which there was progression towards a normal pattern of differentiation. An increase in differentiation was shown by this means to represent increased maturation of cells of the neoplastic process in one study on a myelodysplastic disorder, demonstrating domination of proliferative activity in culture by all of the myelodysplastic disorders examined. Transition towards normal differentiation in nine studies on acute leukaemia, however, correlated with partial or complete replacement of the acute leukaemic cells by normal haemopoietic series in de novo acute leukaemia, and by Ph positive cells in blast crisis of CML. Conversion to morphologically and cytogenetically normal cell populations in five studies on de novo acute leukaemia occurred in four cases which failed to respond to remission-induction therapy, suggesting the selective toxic effect capable of purging acute leukaemic cells from bone marrow operated by a mechanism which lacked cross-resistance to currently-employed cytotoxic agents.