While mycobacteria are unable to degrade lanosterol and dihydrolanosterol, principal components of wool fat, we observed the transformation of some of their autoxidation products by Mycobacterium phlei. By analogy with the mechanism of degradation of cholesterol, this difference was assumed to be due to the requirement for the presence of an enone group before the side-chain can be degraded. This paper reports the spectroscopic determination of the structure of the major metabolites of 7,11-dioxodihydrolanosterol. The side-chain is degraded from eight carbon atoms to three, the terminal carbon atom being oxidized to a primary alcohol or a methyl ester. The tetracyclic skeleton can undergo regioselective oxidation–reduction modifications at the 3- and 7-position. Their conformational analysis, carried out by 2D-NMR methods, indicates a chair form for ring A of 3β-hydroxy derivatives, while it is highly deformed for 3-keto compounds as predicted formerly by Mislow for this lanostane series.