Blood monocytes isolated from healthy human donors were cultivated for 3 days in the presence of recombinant human interferons (rHuIFN) alpha, beta, gamma. Intracellular activity of cathepsin B was recorded. All rHuIFNs suppressed the cathepsin B activity in the monocytes, rHuIFNs alpha and beta suppressed in a dose-dependent manner, whereas rHuIFN-gamma was suppressive only at low concentrations (1-10 U/ml). In parallel experiments, monocytes were stimulated with carrageenan, which caused increased cathepsin B activity in the cells. This increase was reduced to the level of activity in nonstimulated cells by all rHuIFNs. We have previously reported that cathepsin B may be involved in the pathogenesis of rheumatoid arthritis, and the present results may have a bearing on the effects of HuIFNs on the immune system.