Seventeen patients with hairy-cell leukaemia (HCL) and peripheral cytopenias were given human lymphoblastoid interferon (Wellferon), 3 megaunits daily or 6 megaunits on alternate days intramuscularly, for 4-24 weeks. Twelve of the patients had undergone splenectomy, three had no palpable spleen and had therefore not been offered surgery, and two patients with substantial splenomegaly were given interferon (IFN) as treatment of first choice. Toxic effects were minor except in one patient who experienced a severe form of somnolence syndrome. In all patients hairy cells (HCs) were cleared from the blood and platelet and Hb levels improved in 2-14 weeks. Neutrophils were improved in 14/17 of the patients. In the two patients with splenomegaly, the spleen became impalpable after 5-8 weeks therapy, and haematological improvement occurred at 12-14 weeks. HC infiltration of the marrow was reduced in all patients, but was complete (less than 5%) in only two, both of whom had impalpable spleens. Immunological surface-marker studies confirmed that light-chain-restricted B cells disappeared from the blood in parallel with the clearance of morphological HCs. There was no evidence of HC maturation and no increase in phenotypic NK cells. T cells were moderately reduced and the relatively greater reduction of Leu 2a+ suppressor cells resulted in increased Leu 3a+/2a+ helper/suppressor ratios in 11/17 of the patients. Early experience in the six patients who have stopped IFN suggests that, after an initial further increase in Hb and neutrophil levels, HCs gradually return with slow deterioration of haematological parameters. Interferon is now the treatment of choice for patients becoming cytopenic post-splenectomy or for patients without splenomegaly. IFN is effective first-line therapy in patients with splenomegaly, but further work is needed to establish whether the agent should replace splenectomy in such patients. Some form of maintenance or re-treatment therapy will probably be necessary.