TNF is synthesized as a 26-kD membrane-anchored precursor and is proteolytically processed at the cell surface to yield the mature secreted 17-kD polypeptide. The 80-kD tumor necrosis factor (TNF) receptor (TNFR80) is also proteolytically cleaved at the cell surface (shed), releasing a soluble ligand-binding receptor fragment. Since processing of TNF and TNFR80 occurs concurrently in activated T cells, we asked whether a common protease may be involved. Here, we present evidence that a recently described inhibitor of TNF processing N-(D,L-[2-(hydroxyaminocarbonyl)methyl]-4-methylpentanoyl)L- 3-(2'naphthyl)- alanyl-L-alanine, 2-aminoethyl amide (TAPI) also blocks shedding of TNFR80, suggesting that these processes may be coordinately regulated during T cell activation. In addition, studies of murine fibroblasts transfected with human TNFR80, or a cytoplasmic deletion form of TNFR80, reveal that inhibition of TNFR80 shedding by TAPI is independent of receptor phosphorylation and does not require the receptor cytoplasmic domain.