In this study we report the expression pattern of 13 different function-associated surface molecules on synovial fluid and peripheral blood granulocytes from rheumatoid and reactive arthritis patients. We found increased expression of the complement receptors 1 (CD35) and 3 (CD11b) and of the activation-associated antigens CD67, CD24, and M5 on synovial fluid granulocytes from rheumatoid and/or reactive arthritis patients compared to autologous peripheral blood granulocytes. In addition, synovial fluid granulocytes expressed IgG Fc receptor 1 (CD64) and complement receptor 4 (CD11c), neither of which can be found on peripheral blood granulocytes. Peripheral blood granulocytes from rheumatoid and reactive arthritis patients expressed higher levels of leucocyte function-associated antigen 1 (CD11a) and of the membrane proteins CD31, CD24, M5, and M6 compared to peripheral blood granulocytes from healthy controls and patients with degenerative joint disease. No significant differences in the expression of any of the molecules studied could be observed between cells from rheumatoid and cells from reactive arthritis patients, suggesting a similar activation process for granulocytes in these two diseases.