The effects of a range of metallopeptidase inhibitors on the activities of the porcine kidney cell surface zinc aminopeptidases, aminopeptidase A (AP-A; EC 3.4.11.2), aminopeptidase N (AP-N; EC 3.4.11.7) and aminopeptidase W (AP-W; EC 3.4.11.16), have been directly compared. Amastatin and probestin were effective against all three aminopeptidases, with the concentration of inhibitor required to cause 50% inhibition (I₅₀) in the low micromolar range (I₅₀ = 1.5–20 μM), except for probestin with AP-N which displayed an I₅₀ of 50 nM. Actinonin failed to inhibit significantly either AP-A or AP-W, and thus can be considered a relatively selective inhibitor (I₅₀ = 2.0 μM of AP-N. In contrast, bestatin was a relatively poor inhibitor of AP-N (I₅₀ = 89 μm) and failed to inhibit AP-A, but was more potent towards AP-W (I₅₀ = 7.9 μM). Thus, some of the observed chemotherapeutic actions of bestatin may be due to inhibition of cell-surface AP-W. A number of other metallopeptidase inhibitors, including inhibitors of endopeptidase-24.11 (EC 3.4.24.11) and membrane dipeptidase (EC 3.4.13.11), and the carboxylalkyl and phosphoryl inhibitors of angiotensin converting enzyme (EC 3.4.15.1) failed to inhibit significantly AP-A, AP-N or AP-W. However, AP-W was inhibited with I₅₀ values in the micromolar range by the sulphydryl converting enzyme inhibitors rentiapril (I₅₀ = 1.6 μM), zofenoprilat (I₅₀ = 7.0 μM) and YS 980 (I₅₀ = 17.7 μM). Neither AP-A nor AP-N were affected by these sulphydryl compounds. Inhibition of AP-W may account for some of the side effects noted with the clinical use of the sulphydryl converting enzyme inhibitors. The availability of compounds which are totally selective for AP-W over any of the other mammalian cell surface zinc aminopeptidases may aid in identifying endogenous substrates, and thus physiological or pathophysiological role(s) of AP-W.