Expression of class II major histocompatibility complex (MHC) on monocytes is a prerequisite for effective antigen presentation and processing, an important component of the immune response to infection. It has been reported that the level of monocyte class II expression may identify patients who go on to develop infective complications following trauma. In the present study, flow cytometry was used to measure MHC class II (human leucocyte antigen (HLA)-DR) expression on circulating monocytes and T cells in 36 patients undergoing elective major resectional surgery, of whom 12 developed septic complications. The percentage of HLA-DR positive monocytes fell significantly on the first day after operation in both groups (P < 0.001) but was significantly higher in those without than in those with sepsis on days 1, 3 and 5 (P < 0.05). In contrast, the level of T cell HLA-DR expression rose significantly on the first day after operation (P < 0.05) in patients without sepsis to a level higher than in those who developed infection (P < 0.05). These findings have important implications, as predictive biological elements and for biological response modification, in patients at risk of developing sepsis after surgery.