Bovine α-chymotrypsin, δ-chymotrypsin, homoarginine-δ-chymotrypsin, and bovine chymotrypsin B were inactivated by nitrous acid at pH 3.8–4.4 and 0°. The potential activity of bovine chymotrypsinogen A was not affected under these conditions. The inactivation rates as measured with the substrates α-N-acetyl-L-tyrosine ethyl ester, carbobenzoxyglycine p-nitrophenyl ester, and p-nitrophenyl acetate, and by 3H-di-isopropyl phosphorofluoridate incorporation were identical with the deamination rates of the amino group of the N-terminal isoleucine-16, but were slower than the deamination rates of the amino groups of the N-terminals half-cystine-1 and alanine-149. It is concluded that the deamination of isoleucine-16 is directly responsible for the inactivation. Third-order deamination rate constants of the N-terminal isoleucine-16 were measured and the following values (in min−1M−2) were obtained: α-chymotrypsin, 0.4–0.6; homoarginine-δ-chymotrypsin, 0.05; di-isopropyl phosphoryl-α-chymotrypsin, [Formula: see text]; tosyl-α-chymotrypsin, 0.05; chymotrypsin B, 0.3; α-chymotrypsin in guanidinium chloride, 30–50; homoarginine-δ-chymotrypsin in guanidinium chloride, > 20. The deamination rate constants for the model dipeptides isoleucylvaline and valylvaline are 40 and 46, respectively (Kurosky, A., and Hofmann, T.: to be published). A comparison shows that the constants for the dipeptides and the two chymotrypsins in guanidinium chloride are very close and are probably those of a fully exposed amino group. The much lower constants for the other enzymes and derivatives represent the varying degrees of accessibility of the amino group and show the usefulness of the reagent as a conformational probe. The results are fully compatible with the proposed structure of α-chymotrypsin (1) and the proposed function of the N-terminal isoleucine (2).