Leishmania amazonensis was found to release nucleoside diphosphate kinase (NdK)-a stable enzyme capable of decreasing extracellular ATP. The release of this enzyme from Leishmania results in its progressive accumulation extracellularly as they replicate, peaking at the stationary phase in vitro. The released NdK is immunoprecipitable and constitutes approximately 40% of its total activities and proteins. The retention of a known cytosolic protein by wild type cells and a fluorescent protein by DsRed transfectants at stationary phase, which release NdK, indicates that this is a spontaneous event, independent of inadvertent cytolysis. Recombinant products of Leishmania NdK prepared were enzymatically and immunologically active. Both recombinant and native Leishmania NdK utilized ATP to produce expected nucleoside triphosphates in the presence of nucleoside diphosphates in excess. Both native and recombinant Leishmania NdK were also found to prevent ATP-induced cytolysis of J774 macrophages in vitro, as determined by assays for lactate dehydrogenase release from these cells and for their mitochondrial membrane potential changes. The results obtained thus suggest that Leishmania NdK not only serves its normal house-keeping and other important functions true to all cells, but also prevents ATP-mediated lysis of macrophages, thereby preserving the integrity of the host cells to the benefit of the parasite.