Mycobacterium leprae in infected armadillo tissue produces extracellular phthiocerol-containing lipids in amounts well in excess of the bacterial mass. The principal component (1.38 mg in 1 g of liver, wet weight, containing 3.7 X 10(10) M. leprae bacilli) consists of a mixture of two phthiocerol homologs, 3-methoxyl-4-methyl-9, 11-dihydroxyoctacosane and 3-methoxyl-4-methyl-9, 11-dihydroxytriacontane, (formula: see text); in which the hydroxyl functions are acylated by a mixture of three 'mycocerosic acids': 2,4,6,8-tetramethylhexacosanoate, 2,4,6,8-tetramethyloctacosanoate, and 2,4,6,8-tetramethyltriacontanoate. The structures were established by saponification of the native lipid, direct probe electron impact- or chemical ionization-mass spectrometry of the phthiocerol or its permethylated derivative, and gas-liquid chromatography-electron impact-mass spectrometry of the methyl esters of the fatty acids. In addition to the previously reported M. leprae-specific triglycosylphenolicdiacyl phthiocerol (Hunter, S. W., Fujiwara, T., and Brennan, P. J. (1982) J. Biol. Chem. 257, 15072-15078), the extracellular products contain small amounts (about 60 micrograms/g of infected liver, wet weight) of two other phenolic glycolipids, one of which (Phenolic Glycolipid III) has been structurally elucidated, (formula: see text); assuming certain enantiomeric configurations for the sugar substituents; the R-acyl functions are identical with those in the diacylphthiocerol. Phenolic Glycolipid-III reacts in enzyme-linked immunosorbent assays with sera from patients with leprosy and with rabbit antisera raised against whole M. leprae. The phthiocerol-containing lipids may be synonymous with the electron transparent capsules of M. leprae, and their unreactive state may confer on them the role of passive protectors of the bacillus.