Differences in the action of adenosine phosphates on the release of intracellular enzymes from human and rat lymphocytes have been studied. The protective effect of ATP on the human cells was found to be less than on the rat cells. The greatest discrepancy was exhibited by AMP which exerted a protective effect on human lymphocytes, but increased enzyme efflux from rat lymphocytes. The activities of adenosine kinase, adenylate kinase, phosphoribosyl-pyrophosphate synthetase, pyruvate kinase, phosphoglycerate kinase and creatine kinase were compared in the cells of both species. Although significant differences were observed, they were too small to suggest the presence of a mechanism for the conversion of AMP into ATP in human lymphocytes not found in the rat cells. It seems therefore that the protective effect exerted by AMP on the human cells is not mediated by its conversion into ATP, and hence that some factor other than the intracellular energy content is concerned in controlling the release of intracellular enzymes from human cells.