Tumor necrosis factor-α (TNF-α) is a potent cytokine in inflammatory processes. A variety of mechanisms that modulate its activity have been described, one being its binding to soluble receptors (sTNFR). In this study, we demonstrate that human monocytic cells such as THP-1 respond to direct contact with a membrane preparation of stimulated HUT-78 cells by producing TNF-α and by releasing sTNFR-p75, but not sTNFR-p55, with different kinetics. TNF-α concentration peaked after 12 h of contact and then decreased, whereas sTNFR-p75 production increased progressively upon cell/cell contact. The decrease in TNF-α concentration is not due to trapping of TNF-α by its soluble receptors or other soluble or cell-associated molecules, but rather to a proteolytic activity associated to THP-1 cells. On the other hand, the increase in sTNFR-p75 release does not result from an increase in the cleavage of pre-existing cell-associated sTNFR-p75 but from an increase in TNFR-p75 expression, immediately followed by the cleavage of its extracellular domain. Phenylmethylsulfonylfluoride, a serine protease inhibitor, has a negative effect on both TNF-α degradation and sTNFR-p75 release by THP-1 cells. Thus, there may be an enzymatic activity associated to THP-1 cells that plays an important role in the neutralization of TNF-α activity both by degrading the molecule and by cleaving its receptors at the cell surface.