Histidine decarboxylase from Lactobacillus 30a is rapidly and irreversibly inactivated upon incubation with L-histidine methyl ester. The rate of inactivation is first-order with respect to remaining active enzyme and exhibits saturation kinetics with a kinact of 1.2 mM and an apparent first-order rate constant of 0.346 min-1 at pH 4.8 and 25 degrees C. On complete inactivation, 3 mol of [14C]histidine (from L-[14C]histidine methyl ester) and 2 mol of 14C (from L-histidine [14C]methyl ester) are bound in nondialyzable form per mol (190 000 g) of protein inactivated with a corresponding loss of three of the five DTNB-titratable--SH groups that are essential for activity of the native enzyme. Imidazole propionate, a competitive inhibitor of the enzyme, protects against inactivation, loss of --SH groups, and incorporation of radioactivity from both the histidine and the methyl ester moieties of the labeled inhibitor, and kinetic evidence indicates that imidazole propionate and histidine methyl ester compete for binding at the active site of histidine decarboxylase in a mutually exclusive manner. Treatment of the labeled protein with either alkali or hydroxylamine results in the quantitative release of radioactivity. These data suggest that inactivation of histidine decarboxylase by L-histidine methyl ester results from two different modes of interaction between the inhibitor and the active site of histidine decarboxylase; the major interaction involves an essential -SH group.