The possibility that serum from hairy-cell leukaemia (HCL) patients at diagnosis may show an inhibitory effect on the in vitro colony growth of normal haemopoietic progenitor cells has been suggested. Several studies have documented the efficacy of alpha-Interferon (alpha-IFN) in inducing a complete restoration of peripheral blood values and, in some cases, a complete clinical remission. In this study we have evaluated the regulatory effect of serum, collected before and after 3 and 12 months of alpha-IFN treatment, from 10 patients with untreated HCL, on the in vitro growth of normal bone marrow CFU-GM, BFU-E and CFU-MK. The effect of conditioned media, prepared from enriched hairy cells (HC) cultured in synthetic medium, on the growth of normal haemopoietic progenitors was also investigated. The results obtained confirm that sera from untreated HCL patients display a variable degree of inhibitory activity in the progenitor cell compartments analysed. Disappearance of the inhibitory activity, particularly evident for the erythroid compartment, was found only in patients who displayed a disappearance of circulating HC and a good haematological response after prolonged (12 months) treatment with alpha-IFN. The possibility that the serum of patients with HCL may contain a haemopoietic inhibitory factor, released by the neoplastic HC population, is suggested.