A single intravenous injection of freshly heparinized blood from a donor-specific blood transfusion (DST) seven days before transplantation significantly prolongs the subsequent survival of hepatic allografts from ACI(RT1a) to LEW(RT11) rats. We used W3/25 (anti-CD4) and OX22 (anti-CD45 RC: an isoform of leukocyte-common antigen [CD45R]) monoclonal antibodies to investigate the cellular identity of hepatic allograft infiltrates following transplantation. The number of CD4+ and CD45RC+ cells in untreated allografts increased equally over time by day seven. However, in DST-treated hepatic allografts, CD4+ and CD45RC+ cells also increased over time by day 14, but the increment in the number of CD4+ cells was significantly greater than that in CD45RC+ cells. While the number of CD4+ cells remained persistently elevated in the hepatic allografts of rats pretreated with DST, they did not initiate rejection. Fluorescence-activated cell sorter (FACS) analysis revealed that the accumulated CD4+ T cells could be divided into two subsets, CD45RCCD4+ and CD45RC+ CD4+ T cells, and that the ratio of CD45RCCD4+/CD45RC+ CD4+ T cells in the hepatic allografts of recipients pretreated with DST was significantly greater than that in untreated allografts. Reverse-transcriptase polymerase chain reaction (RT-PCR) analysis demonstrated that CD45RC-CD4+ T cells expressed interleukin (IL)-4 and IL-10 messenger RNA (mRNA), but not IL-2 and interferon gamma (IFN-γ). The pattern of messenger RNA expression in hepatic allograft infiltrates from animals pretreated with DST provides compelling evidence for the selective in vivo preservation of T-helper (Th2)-specific cytokines in the rat system. Our studies show that CD45RC leukocyte-common antigen expression can define different populations of hepatic infiltrating CD4+ T cells. A persistent infiltration of CD45RCCD4+ T cells, Th2-like effector cells, is characteristic of hepatic allografts with a prolonged survival in DST-pretreated rats.