Survival times of allogeneic fetal pancreases were determined across various immunogenetic barriers using several inbred rat strains. Pancreases from 17-day-old embryos transplanted beneath the kidney capsule of normal, nonimmunosuppressed recipients were richly vascularized within a short period regardless of histoincompatibilities. The transplants grew progressively, increased beta cell number, and synthesized insulin at a rate similar to that in isografts until rejection intervened. Survival end points were scored by (1) complete disappearance of intrinsic vascularization and necrotic change of tissue by gross observation of grafts, (2) a sharp decrease in insulin content in a graft as compared to that in a control isograft, and (3) destruction and disappearance of beta cells in histological examination. Although fetal pancreases were rejected within 10 days in all strain combinations, there were clear differences in host immune-reactions in terms of immunogenetic barriers between donor and recipient.