The reaction of equimolar amounts of N-bromosuccinimide and hen egg-white lysozyme in acetate buffer, under the conditions of Hayashi et al. (Hayashi, K., Imoto, T., Funatsu, G. and Funatsu, M. (1965), J. Biochem. (Tokyo) 58, 227), yields a protein mixture that has a time-dependent 13C-NMR spectrum. The initial natural-abundance 13C-NMR spectrum indicates the presence of about equal amounts of [oxindolealanine-62]lysozyme and [δ1-acetoxytryptophan-62] lysozyme. The latter converts to [oxindolealanine-62] lysozyme w ith a half-life of about 2 days at 25 °C and pH 3.9. Two observations indicate that the source of the acetylgroup of δ1-acetoxytryptophan-62 is the acetate buffer. First, the spectrum of a lysozyme sample treated with N-bromosuccinimide in the presence of [1-13C]acetate yields a verystrong acetyl ester carbonyl resonance. The time dependence of the intensity of this resonance yields a half-life of 44 h for [δ1-acetoxytryptophan-62]lysozyme. Second, the initial natural-abundance 13C-NMR spectrum of a lysozyme sample treated with A′-bromosuccinimide in the absence of acetate indicates essentially complete conversion of tryptophan-62 into oxindolealanine.