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The African clawed frog, Xenopus, is a widely used comparative model for studying the immune response to transplantation antigens.To better define the effector cells involved in the immune response to skin alloantigens of the frog Xenopus laevis, we have adapted a whole-mount immunohistology procedure used in mice that enables us to visualize leukocyte infiltration into unfixed transplanted skin tissues using fluorescent antibodies. We characterized the leukocyte populations present in donor skin at different times after transplantation using anti-class II and CD8 monoclonal antibodies.In autografts, only class II+ Langerhans or dendritic-like cells and very few CD8+ T cells were detected. In contrast, major histocompatibility complex (MHC) disparate skin grafts at the peak of acute rejection (seven days posttransplantation, 50% rejection of pigment cells) were infiltrated with a large number of bright class II+ leukocytes, the majority of which were CD8+ T cells. Most of these cells were located outside blood vessels and often near areas lacking pigmentation. Compared to MHC-disparate skin grafts, skin differing from the host only by minor histocompatibility antigens undergoes slower (i.e., chronic) rejection; interestingly, however, it was infiltrated by similar numbers of class II+ and CD8+ T cell effectors, but with delayed kinetics (i.e., peaked around 15 days posttransplantation).Our data provide direct in vivo evidence of marked infiltration of effector leukocytes, a majority of which are CD8+ T cells that occurs at the onset of tissue destruction of skin allografts.