The anaphylatoxin C5a is a potent proinflammatory stimulus with immunomodulatory activities. Expression of its receptor C5aR (CD88) has been detected on cells of myeloid origin such as granulocytes and monocytes/macrophages. However, controversial results exist on the expression of C5aR on T and B lymphocytes as well as on mature dendritic cells (DC). The aim of the present study was to characterize expression of C5aR protein on myeloid and lymphoid cells in the mouse. For this purpose, rat monoclonal antibodies with specificity against the murine C5aR were generated. Using these reagents a distinct amount of C5aR antigen was observed on neutrophils and macrophages. In contrast, C5aR protein was not detectable on resting or stimulated murine T or B lymphocytes. Furthermore, no C5aR protein could be observed on splenic CD11c positive DC which have been classified in the literature as relatively mature. Taken together, our results suggest that in the mouse expression of C5aR protein may be restricted to leukocytes of myeloid origin whereas previous evidence for C5aR expression on lymphoid cells may be reevaluated.