Although the complement system is implicated in the inflammatory process in arthritic diseases, a direct interaction between chondrocytes and complement has not been demonstrated. In this study, we investigated expression of the C5a receptor (C5aR) on chondrocytes of cartilage from patients with rheumatoid arthritis (RA), osteoarthritis (OA), and bone fracture as normal controls by reverse transcriptase polymerase chain reaction (RT-PCR), flow cytometry, and immunohistochemistry. The RT-PCR detected mRNA for C5aR in most or all of the tested samples (73% in OA, 100% in RA, 89% in normal). The FACS analysis revealed different expression ratios between individuals varying from 0.7% to 77.1%; however, expression ratios of C5aR were significantly higher in RA than in controls (26.0% in RA, 9.0% in OA, 6.9% in normal). The expression of C5aR was upregulated significantly by addition of IL-1beta in RA and normal samples but not in OA. In addition, the C5aR-positive chondrocytes were confirmed by immunohistochemistry. In conclusion, expression of C5aR and the effect of IL-1beta on the expression were different between RA and OA. The C5aR may contribute to chondrocyte metabolism and the pathogenesis of arthritis differently between in RA and OA.