Purified protein methylesterase (PME) from rat kidneys was incubated with ovalbumin-methyl esters and a series of protease inhibitors. All four inhibitors with C-terminal aldehyde, leupeptin, chymostatin, Boc-Gln-Leu-Lys-H and D-Phe-Pro-Arg-H completely blocked PME activity. Other inhibitors including, alpha-1 antitrypsin, soybean trypsin inhibitor, antithrombin III, phenylmethylsulfonylfluoride, aprotinin and lima bean trypsin inhibitor had no significant effect whereas pepstatin, at high concentration reduced the enzymatic activity by 25%. The most potent inhibitors, leupeptin and chymostatin, had a Ki of 3.5 X 10(-8) and 5.4 X 10(-7) M, respectively. These inhibitors provide two new tools to study PME function.