Mycobacterium bovis (BCG) was submitted to lipid extraction involving gentle and drastic procedures. Neutral lipids, glycolipids as well as phospholipids were detected in the diethyl ether extract, whereas the chloroform and chloroform-methanol extracts contained mainly phospholipids. Arabinose mycolates, which represent firmly bound cell wall lipids, were detected in the acidified ether extract. The lipid extracts were adsorbed to charcoal particles and inoculated intravenously into mice. Histological analysis showed that the particles were trapped in the lung microcirculation and strong inflammatory reactions were induced around the charcoal particle coated lipids present in the diethyl ether, chloroform and acidified diethyl ether extracts. Fractionation of these inflammation-inducing extracts by column and preparative thin-layer chromatography led to the isolation of five active mycolic acid-containing glycolipids, identified as glycerol monomycolate, glucose monomycolate, arabinose monomycolate, trehalose monomycolate and trehalose dimycolate on the basis of physical, chemical and spectroscopic analyses. Inflammatory reactions similar to that observed after inoculation of live BCG were induced in the lungs by trehalose monomycolate, trehalose dimycolate or glucose monomycolate. However, the toxic reactions caused by glycerol monomycolate and arabinose monomycolate were characterized by an acute inflammatory process mainly due to the constant and massive presence of polymorphonuclear leukocytes.