Schistosomiasis japonica in capuchin monkeys (Cebus apella) and schistosomiasis mansoni in baboons (Papio cyanocephalus and P. hamadryas) were completely arrested for 6 months in every infected primate receiving a single treatment with tubercidin (Tu), administered after prior absorption into 20% of their red cells. It is very likely that a single treatment with Tu sequestered in only 15% of the hosts' red cells would also be 100% effective for prolonged periods of time, but that with lower doses some relapses would be expected. Baboons with patent Schistosoma mansoni infections were rechallenged with S. mansoni cercariae 4 months after treatment with Tu. Although Tu eliminated almost all the sexually mature female worms from the primary infection but spared most of the males for continuing sojourn within their hosts, the baboons retained their full susceptibility to reinfection, as indicated by worm burdens and fecal egg excretion. However, the granulomatous reaction in the rechallenged Tu-treated baboons to new masses of eggs trapped in their livers appeared to be less intense than was seen in animals with primary infections.