Fairly pure leprosy bacilli were easily collected from nude mouse foot pad lepromas by the Ficoll density gradient centrifugation and alkali treatment methods. The yield of bacilli available for biochemical study was 42.6%. The density of Mycobacterium leprae was very heterogeneous. The percent of solid bacilli in the light bacilli fraction was 23%; that in the heavy bacilli fraction was 40%. The endogenous respiration activity in the heavy bacilli was greater than that in light bacilli. The average coefficient of respiration in M. leprae was 1 microliter O2/mg X hr. In the whole cells of M. leprae, a cytochrome b1 absorption peak and its Soret peak were detected at wavelengths of 560 nm and 426 nm, respectively. However, a cytochrome a2-like peak (which was observed in M. lepraemurium), and a cyt c and cyt a were not detected. Catalase activity was not found in whole cells, the cell-free extract, or particle fractions of M. leprae. Any catalase activity associated with M. leprae suspensions is a tissue contaminant. NAD-peroxidase activity was also not detected in the cell-free extract of the leprosy bacillus. These results would indicate that leprosy bacilli cannot degrade hydrogen peroxide.