1. A new assay of graft-host interactions has been presented. It consists of the early finding of liver infiltrates in responsive and unresponsive homologous recipients 1 to 3 days after the intravenous injection of competent lymphoid tissue in the mouse. 2. Evidence is presented that infiltrates are of donor origin, and are influenced by a homologous antigenic environment. They can be quantitatively increased by preimmunization of donor to recipient and decreased by preimmunization of recipient to donor. Furthermore, they are present in lethally x-irradiated or neonatally thymectomized unresponsive recipients after intravenous injection of competent homologous spleen cells. 3. Splenomegaly, which also occurs in this early period, is a product of both donor and host-specific immune reactivity together with a small non-specific component of host reactivity. 4. A comparison, of the responsiveness of lymphoid tissue from different strains to the same transplantation antigens is presented, together with the observation of differences in the reactivity of reciprocal donor-recipient injection combinations to each other. 5. Graft-host interactions with this model can be demonstrated in donor-host strain combinations isogenic at the H-2 locus without preimmunization of the donor. Differences in reactivity of reciprocal donor-recipient combinations of lymphoid tissue injections to each other can be detected in this group as well as in the groups differing at the strong H-2 locus
