We examined a potential mechanism for adoptively transferred resistance against Schistosoma mansoni in schistosome-susceptible snails receiving allografts of the hematopoietic amebocyte-producing organ (APO) from a schistosome-resistant strain of Biomphalaria glabrata. Susceptible NIH albino snails first were implanted with the APO from resistant Salvador strain donors. At 14 days post-implantation, cell-free plasma was isolated from APO recipients and injected into a second group of NIH albino snails. Controls were injected with plasma from NIH albino snails that previously had received implants of Salvador mantle, an immunologically inert tissue. Finally, plasma recipients, along with untampered (non-injected) controls, were exposed to miracidia of S. mansoni and subsequently monitored for infection. A significantly lower prevalence of infection occurred in the group injected with plasma from APO recipients (53%) in comparison with that in untampered controls (100%) or in snails receiving plasma from mantle recipients (97%). These results suggest that adoptively transferred resistance in B. glabrata results from secretion of soluble resistance factor(s) by the implant, rather than, or in addition to, formation of a hemocyte chimera having cytotoxic donor hemocytes.