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Since the application of low doses of IFN-alpha is necessary to maintain remissions in Hairy Cell Leukemia (HCL) it is of interest whether peripheral blood mononuclear cells (MNC) of HCL patients can be induced in vitro to produce IFN-alpha. 9 patients suffering from advanced HCL were included in the study. The diagnoses were confirmed by characteristic findings in peripheral blood and bone marrow biopsies. For IFN treatment we initially used natural IFN-alpha (Bioferon) and switched later to recombinant IFN-alpha2 (Boehringer). MNC of 5 patients before IFN therapy and of 6 patients during IFN therapy (2–47 weeks) were induced by phythemagglutinin (PHA), Corynebacterium parvum (C.p.), and sendai virus (SV). PHA is known to induce IFN-gamma. Both, C.p. and SV induced IFN-alpha but no IFN-gamma in MNC of healthy controls and of IFN treated breast cancer patients. In HCL patients normal antiviral activities could be induced by PHA. Zero or only low antiviral activities could be induced in MNC from 9 patients tested on 22 occasions. It is concluded that MNC from patients with advanced HCL can be induced to produce IFN-gamma but no IFN-alpha. Since IFN-alpha but not IFN-gamma is produced by monocytes it is likely that reduced numbers of monocytes which were found in our HCL patients before and during IFN treatment account for the described deficiency of IFN-alpha production.