We investigated the mechanism of tolerance induction by identifying differences in infiltrating cell population and Ig subclasses deposition between tolerant and rejecting rat liver allografts.There were occasional minor differences in the T cell or CD11 b/c+(monoeyte/macrophage/granulocyte)infiltrate.By contrast,there was a major difference in B cell infiltrate with more B cells in tolerant grafts.IgM was increased in both tolerant and rejecting livers while deposition of IgG1 was observed in rejecting but not in tolerant livers.