Human polymorphonuclear leukocytes (PMNs, 4 x 10(6)/ml) converted human big endothelin (bET) to an endothelin-1 (ET-1)-like contractile factor, as assessed by bioassay. The formation of this ET-1-like activity from bET was partially inhibited by phosphoramidon (54 micrograms/ml), but not by pepstatin-A (1 microgram/ml), epoxysuccinyl-L-leucylamido(guanidino)butane (E-64, 10 micrograms/ml) or phenylmethylsulfonyl fluoride (PMSF, 25 micrograms/ml). In addition, nonactivated PMNs converted [125I]bET to [125I]ET-1, thus confirming the bioassay results. Incubation of ET-1 with fMLP-activated PMNs or cell-free supernatants from activated PMNs resulted in the loss of its contractile activity, and this loss of activity was paralleled by the metabolism of [125I]ET-1. The metabolism of [125I]ET-1 by PMNs or leukocyte cathepsin G (5 micrograms/ml) was prevented by PMSF (25 micrograms/ml), but not by phosphoramidon (54 micrograms/ml) or pepstatin-A (1 microgram/ml). Thus, PMNs can form ET-1 from bET via a neutral protease and degrade ET-1 via a serine protease, an observation that may have important pathophysiologic implications in disease states associated with PMN infiltration.