These data indicate that in rat heterotopic syngeneic SBT, the venous drainage system of the graft has profound effects on cell recovery in the gut-associated lymphoid tissue of both graft and host. Depending on the functional status of the same lymphoid tissue (Gorezynski, personal communication, 1992) one could thus anticipate significant perturbation of host-antigraft (and GVH) reactivity in allogeneic situations according to the venous drainage used. The mechanism(s) responsible for these effects have not been investigated. However, one testable hypothesis is that (a) factor(s) from the gut can control lymphoid recirculation within the mesenteric lymphoid tissue, and that this (these) factor(s) are absorbed under normal circumstances by hepatic tissue.