The yeast ATP synthase subunit d was over-expressed in E. coli and formed inclusion bodies. It was purified by solubilization in urea and slow removal of the urea by stepwise dialysis in the presence of a non-ionic detergent. The resulting soluble subunit d was used to prepare polyclonal antibodies. Blots of yeast mitochondrial proteins were probed with these antibodies. The strain disrupted in ATP4 gene encoding the subunit 4 displayed only 8% of the wild type subunit d. Antibodies against subunit d did not inhibit the wild type ATPase activity.