Previous studies using semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) have demonstrated that islet xenograft rejection in mice is dominated by Th2-associated cytokines, i.e., interleukin (IL)-4 and IL-10. However, immunohistochemical stainings show that the morphological pattern in this model is more reminiscent of a delayed-type hypersensitivity (DTH) reaction, which is associated with a Th1 response. This study was designed to resolve the mechanisms of acute cellular xenograft rejection in rats transplanted with fetal porcine islet-like cell clusters (ICCs). Real-time quantitative RT-PCR was used to quantify the mRNA expression of cytokines in the grafts and lymph nodes, and the findings were related to the immunopathology of the rejecting grafts. By day 1, mRNA expression levels of IL-1 beta, IL-2, IL-12p40, interferon-gamma, and tumor necrosis factor-alpha were already induced in the lymph nodes. From days 3 to 12, an increasing amount of activated macrophages was seen in the grafts, whereas T- and NK-cells were fewer and mainly accumulated in the periphery of the grafts. Most of the ICCs were rejected by day 5. Transcripts of Th1-associated cytokines were dominant in both regional lymph nodes and in the grafts, with peak levels on days 3 and 5, respectively. The mRNA expression of IL-4 was increased on day 12, and it correlated with the infiltration of eosinophils and an increased level of xenoreactive IgG. The data presented indicate that an islet xenograft triggers a sequential activation of 1) a Th1-associated response characterized by graft destruction in a DTH-like reaction and then 2) a subsequent Th2-associated response characterized by increased levels of xenoreactive antibodies.