The design and synthesis of 3-amino-2-oxo-4-phenylbutanoic acid amides (alpha-keto amides), a new class of aminopeptidase inhibitor, are described. These compounds, illustrated by the Phe-Leu analogue 2, are effective inhibitors of arginyl aminopeptidase (Ki = 1.5 microM), cytosol aminopeptidase (Ki = 1.0 microM), and microsomal aminopeptidase (Ki = 2.5 microM). The ketone carbonyl of the alpha-keto amide was found to hydrate readily in an aqueous DMSO solution, due to the electron-withdrawing effect of the neighboring amide group. A mechanism of inhibition is proposed for the alpha-keto amides that is similar to that proposed for the structurally related aminopeptidase inhibitor bestatin and its analogues, wherein the inhibitor may interact with the S1'-S2' subsite of the enzyme rather than the S1-S1' subsite. Like bestatin, the alpha-keto amides are slow-binding inhibitors of all three enzymes.