Retinoids affect the growth and differentiation of haemopoietic cells. Individuals deficient in retinoids become anaemic; replacement therapy with retinoids corrects the anaemia. Retinoids enhance the clonal proliferation of erythroid and myeloid precursors in soft-gel culture; all-trans-retinoic acid and 13-cis-retinoic acid are the most potent. Retinoids also induce the differentiation of HL-60 promyelocytes to functional granulocytes and can induce cells from other relatively mature, myeloid cell lines to undergo partial differentiation. Cells from less mature myeloid leukaemic lines are often resistant to induction of differentiation by retinoids. Like cells from established lines, relatively mature leukaemic cells (promyelocytes, myelomonoblasts) harvested from patients can undergo differentiation in vitro in the presence of retinoids. A few reports suggests that a minority of patients with myeloid leukaemia or preleukaemia who receive 13-cis-retinoic acid will have improvement in their haemopoiesis. Further studies are required to understand the mechanism of action of retinoids on the growth and differentiation of haemopoietic cells and to explore more fully the therapeutic potential of retinoids.