The inactivation rate of synthetic bradykinin (BK) by different samples of crystallized α-chymotrypsin at pH 7.0, 30° and a 1:10 enzyme/substrate molar ratio was reduced in the presence of 1,10-phenanthroline. Selective inaetivation of the samples by preincubation with l-(1-tosylamido-2-phenyl) ethyl-chloromethylketone (TPCK) in 1:10 enzyme/TPCK ratio reduces the inactivation rate about 60 per cent. This residual BK-inactivating action is probably due to a carboxypeptidase-B-type enzyme present in the chymotrypsin commercially available. In a 1:200 enzyme/substrate ratio and in the presence of 1,10-phenanthroline, a few min of incubation at pH 7.0, 30°, is sufficient to inactivate bradykinin at a constant rate. This inaetivation is due to the cleavage of the Phe-Arg bond by chymotrypsin, releasing free arginine. The rate of inactivation of BK by chymotrypsin in the presence of 3.3 × 10−3 M 1,10-phenanthroline was 43.9 nmoles/min/mg of enzyme, and the inactivation rates of the longer kinins. kallidin (lysylbradykinin) and Met-Lys-bradykinin. were 60.1 and 138.18 nmoles/min/mg of enzyme respectively.