A metalloprotease that converts TNF-.alpha. from the 26 kD cell form to the 17 kD form has been isolated and purified and the cDNA sequence known. In particular, the protease has a molecular weight of approximately 80 kD. The isolated and purified protease is useful for designing an inhibitor thereof, and may find use as a therapeutic agent. Assays for detecting the protease-inhibiting activity of a molecule are also an aspect of the invention.