Halobacterium halobium cells grown under semi-anaerobic conditions convert part of their cell membrane into "purple membrane" which contains a rhodopsin-like protein, bacteriorhodopsin. Under anaerobic conditions in the dark the ATP content of such cells decreases sharply. Either light or oxygen restores the ATP content to the original level. The light effect is mediated by the purple membrane. Inhibitors of the respiratory chain abolish the oxygen response but do not affect the light response. Uncouplers, which function as proton translocators, abolish the light response. These results indicate that the purple membrane functions as a lightdriven proton pump and the cells use the resulting chemiosmotic gradient for ATP synthesis.