Abstract A study of the lipids contained in purified preparations of cytochrome c oxidase has been made. Other than the cholate which had been added as a solubilizing agent, only about 0.05 mg of lipid per milligram of protein was present. The phospholipid content did not exceed 9 μg per milligram protein in any preparation analyzed. For the most part, the lipids could be identified with the degradation products of naturally occurring lipid. These results suggest that in the isolated cytochrome c oxidase system, although solubilizing agents are required to achieve high enzyme activity, there is little or no specificity required for phospholipid.