Previously we have demonstrated that islets of Langerhans treated with donor-specific anti-Ia serum and complement survive when transplanted across the major histocompatibility complex of the mouse. In this study, using immunofluorescence, we demonstrate two morphologically distinct populations of Ia-positive cells scattered within the Ia-negative islet tissue. A large irregularly shaped Ia-positive subset of cells were identified as dendritic cells by using the 33D1 antibody specific for a mouse dendritic cell antigen. The other small, round Ia-positive subset was 33D1 negative. Islets pretreated with anti-dendritic cell antibody and complement prior to transplantation survived in their histoincompatible recipients for >200 days. Rejection of stable islet allografts promptly occurred when transplant recipients were challenged with 1 x 105 donor dendritic cells 60 days after transplantation. These results demonstrate an important in vivo role for donor dendritic cells in the stimulation of allograft rejection.