1. The sterol, unsaturated fatty acid and cytochrome contents of cells of a delta-aminolaevulinate synthase mutant of Saccharomyces cerevisiae are manipulated by growing the organism in media containing defined supplements of delta-aminolaevulinate and other porphyrin intermediates. 2. If unsaturated fatty acids are added to the growth medium as Tween 80, sterol content and respiratory cytochromes alone are manipulated. 3. In the presence of delta-aminolaevulinate (10-50mg/1) cells exhibit moderate to high respiratory activity, but growth yields are low, indicating a loss of oxidative phosphorylation. This is associated with the depletion of membrane lipids, either unsaturated fatty acids and sterols together or sterols alone. 4. Sterol depletion leads to the loss of coupled mitochondrial oxidative phosphorylation in vitro. 5. The lesion in oxidative phosphorylation is associated with an increase in the passive permeability of sterol-depleted mitochondria to protons. 6. Arrhenius plots of mitochondrial permeability to protons indicate that the activation energy for proton entry increases as the sterol content of the membranes decreases. 7. Studies on a cytoplasmic petite mutant isolated from strain ole-3, which lacks a functional membrane-bound protein-translocating adenosine triphosphatase, indicate that proton permeability of the petite mitochondria varies as a function of sterol composition in the same way as that of ole-3 grande mitochondria. This indicates that sterols alone are probably directly responsible for the increased proton entry, owing to a reorganization of the lipid in the membrane. 8. Supplemented ole-3 cells with a normal lipid composition and normal or higher than normal respiratory activities have a growth efficiency only 65% of that of the wild-type, indicating that a further lesion in energy metabolism may be present.