This study was undertaken in an initial effort to characterize the immunology of extremity transplantation by examining the pattern and kinetics of leukocyte migration from rat limb transplants. Migration of donor leukocytes was evaluated by examining recipient lymphoid tissues with a donor-specific, anti-major histocompatibility complex, class I monoclonal antibody. Double-antibody, two-color labeling was used to localize donor cells to specific regions within these tissues. Donor leukocytes, with dendritic cell morphology, were found in the T-cell-rich areas of lymph nodes draining the allograft and spleen. The donor cells were present on postoperative days 1 through 3 but were not present on days 5 to 7. Donor leukocytes were not present in distant lymph nodes or liver. These findings indicate a migration of leukocytes, most likely the highly immunogenic dendritic cell, from rat limb transplants to the draining lymphoid tissues. Migration occurs shortly after transplantation and may lead to the sensitization of alloreactive T-cells.