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N-Bromosuccinimide (NBS) is a known protein reagent able to modify amino acids and proteins, resulting in oxidation of tryptophan, tyrosine and histidine residues, as well as sulfhydryl, alcohol and phenol groups. These properties make NBS a suitable reagent to selectively block certain amino acid residues in biochemistry, and also permit the histochemical detection of proteins by oxidative deamination followed by the Schiff reaction. In this paper we show that, under ultraviolet excitation, NBS selectively reveals the cytoplasmic granules of mammalian eosinophils and chicken heterophils, rendering considerable white–blue fluorescence, in a remarkable fluorogenic reaction which rapidly increases at the beginning of the observation. This emission slightly decays afterwards and then remains almost stable still yielding a high level of emission after 10 min of continuous excitation. Possible mechanisms underlying these results are discussed and we propose NBS as a very suitable fluorogenic reagent for the microscopical detection and analysis of proteins.