The two tumor necrosis factor receptors (TNF-Rs) exist in their soluble form in different biological fluids. In this study we investigated the concentrations of soluble tumor necrosis factor receptors (sTNF-Rs) in the culture supernatants of leukemic cells and in the serum obtained from 33 patients: 12 with hairy cell leukemia (HCL) and 21 with B cell chronic lymphocytic leukemia (B-CLL). In seven patients with HCL, sTNF-Rs were also evaluated following in vivo treatment with interferon-alpha (IFN-alpha). Purified leukemic cells from patients with HCL and B-CLL spontaneously released sTNF-R75 but not sTNF-R55. The levels of sTNF-R75 were higher in supernatants obtained from cultured hairy cells than from cultures of B-CLL cells. The shedding of sTNF-R75 was further increased both in HCL and B-CLL subjects by some B cell-related stimuli, including BCGF, PMA, SAC and was partially inhibited by IFN-alpha in patients with HCL. Sera from HCL patients presented increased levels of both sTNF-Rs with respect to normal controls. Treatment of HCL patients with IFN-alpha resulted in a decrease in serum levels of sTNF-Rs, particularly sTNF-R75. These findings suggest that leukemic cells account for the increased serum levels of sTNF-R75 observed in patients with HCL and B-CLL, but not for sTNF-R55.