The photoreactive arylsulfenyl chloride 2‐nitro‐4‐azidophenylsulfenyl chloride (2,4‐NAPS‐Cl) has been used for the selective modification of tryptophan in Kunitz's soybean trypsin inhibitor (SBTI). The ultraviolet absorption spectrum and amino acid analysis of 2,4‐NAPS‐SBTI indicated that only one of the two tryptophans (93 or 117) present in SBTI was modified. CNBr cleavage of 2,4‐NAPS‐SBTI resulted in two fragments 1–114 and 115–181.Amino acid analysis of the two separated fragments showed that only tryptophan 93 underwent modification. 2,4‐NAPS‐SBTI fully retained its inhibitory activity against trypsin. The photoaffinity labeling of trypsin with 2,4‐NAPS‐Cl was performed on tritiated trypsin prepared by reacting bovine trypsin with [3H]‐succinimidyl propionate. The covalent attachment of 2,4‐NAPS‐SBTI to the tritiated trypsin after photolysis was demonstrated by exclusion chromatography on Sephadex G‐50 in th