Background: Aminophenyl fluorescein was developed to detect the halogenating activity of myeloperoxidase. Results: Aminophenyl fluorescein can also be used to quantify the formation of HOBr by eosinophil peroxidase. Conclusion: Aminophenyl fluorescein is suitable to evaluate the halogenating activity of eosinophil peroxidase and peroxidase activity modulators. Significance: Eosinophil peroxidase, a mammal heme peroxidases family member, may contribute to the regulation of inflammation.