Xanthine phosphoribosyltransferase (XPRTase; EC 2.4.4.22) was found in the promastigotes of four species of Leishmania (L. mexicana, L. donovani, L. braziliensis and L. tarentolae). In no case was there any transribosylation from 5-phosphoribosyl-1-pyrophosphate (PRibPP), forming XMP, in dialyzed preparations, unless activated by a divalent cation. Magnesium and zinc were very low in activation efficiency in all cases, while manganese was optimally efficient. Cobalt was essentially equal to manganese for activation of the enzyme from L. mexicana and L. braziliensis but much less efficient for the enzyme from L. donovani and L. tarentolae. Gel filtration profiles of cell extracts of L. mexicana on Sephadex G-200 indicated that the enzymes catalyzing the transribosylation from PRibPP to guanine, hypoxanthine, and xanthine were inseparable. All were eluted near the void volume. The enzyme for adenine transribosylation was clearly separate. When cell extracts of L. mexicana were applied to Sephadex G-100 columns, the activity toward XMP formation from xanthine eluted with the void volume, together with a portion of that for the formation of GMP and IMP from guanine and hypoxanthine. A second peak of HGPRTase (EC 2.4.2.8) eluted somewhat later and was devoid of XPRTase activity. XPRTase from promastigotes of L. mexicana is heat labile, has rather a broad pH optima, and is stable to freezing when protected by nonspecific cell protein (40,000 g supernate as opposed to 100,000 g supernates).