The fluorescein diacetate-ethidium bromide (FDA/EB) staining method for determining the viability of mycobacterial cells reported by Kvach and Veras was modified for applying the method to Mycobacterium leprae and Mycobacterium lepraemurium. Our method clearly differentiated green-stained cells from red-orange ones with either mycobacterial species and permitted their original fluorescence for over 4 min to be maintained.