Baboons (Papio cyanocephalus and P. anubis), infected with Schistosoma mansoni or S. japonicum, were treated with single doses of tubercidin (7-deazaadenosine; Tu), 1, 3, and 5 mg per kg of body weight, administered by intravenous drip. Crystalline Tu was dissolved in sterile 0.9% NaCl solution (1 mg per ml), and the solution was delivered at a rate of 4 ml per minute. Detectable short-term host toxicity was limited to the 5 mg per kg dose, mainly in the form of reversible mild to moderate kidney damage. Only this 5 mg per kg dose administered to baboons with relatively heavy S. mansoni infections was capable of completely suppressing fecal egg excretion for 6 to 8 weeks, eliminating the female worms, and terminating active disease, as indicated by histopathological findings. Comparable effects were achieved following the administration of the 3 mg per kg dose to baboons with moderate to heavy S. japonicum infections.