Cardiopulmonary bypass is usually associated with an increased risk of post-operative infections and systemic inflammatory response syndrome. This is accompanied by a neutrophil leucocytosis and lymphopenia. Less is known about the role of monocytes and markers of monocyte activity. This study focuses on the changes of Fc-gamma receptors on monocytes in patients undergoing on-pump coronary artery bypass grafting (CABG).The surface expression of CD64, CD32 and CD16 were studied using flow cytometry in 37 patients scheduled for CABG. The antigen density and the percentage positive cells were monitored preoperatively and on day 1, 2, 3 and 7 postoperatively. CD64 and CD32 antigen density were significantly increased from day 1 to day 7(p<0.0001). A significant increase (p<0.0001) in the percentage CD16+ monocytes was detected at day 1,2,and 3. The downregulation of CD16 expression on day 1 was followed be elevation at days 2,3(p<0.01). On day 7th percentage CD16+ monocytes and density were not returned to baseline values. Only the baseline levels of CD64 was lower compared to controls(p<0.05). The results suggest that on-pump CABG induces dynamic changes in the expression of Fc-gamma receptors on monocytes as late as 7 days. We observed significant upregulation in the expression of CD64 and CD32 and “to phases” distribution of CD16 in the post-CABG period.