Phthiocerol dimycocerosates and related compounds are important mole-cules in the biology of Mycobacterium tuberculosis, playing a key role in the permeability barrier and in pathogenicity. Both phthiocerol dimyco-cerosates, the major compounds, and phthiodiolone dimycocerosates, the minor constituents, are found in the cell envelope ofM. tuberculosis, but their specific roles in the biology of the tubercle bacillus have not been established yet.