The attractiveness of baboons as animal models for the study of immunological aspects of schistosomiasis mansoni has rested upon two assumptions: 1) that although they can become immune, they do so only with comparative difficulty, and 2) that this situation presents a close analogy to that in man. The evidences upon which these assumptions have been based are either poorly documented, fragmentary, or circumstantial. The rhesus monkey is known to develop a strong immunity to a percutaneous challenge infection after surgical introduction (homologous transfer) of adult worms into the hepatic portal system. Four experiments of this nature were performed in baboons. The pooled results (9 transferred-challenged experimental and 5 untransferred-challenged control animals) showed that no immunity was conferred by surgical implantation of 80 worm pairs followed by percutaneous exposure 2 to 3 months later to 2,000 cercariae. Worm burdens and fecal egg excretion were the criteria of immunity applied. One control animal died, presumably of schistosomiasis, but circumstances suggest that the transferred worms did not protect against a lethal challenge. There were also no significant differences in tissue egg loads or egg deposition sites along the intestinal tract between control and experimental animals. Thus, the earlier observations suggesting a real difference in the immune capabilities of baboons and rhesus monkeys against Schistosoma mansoni find support in the present experiments.