BACKGROUND The proposed immunosuppressive effect of blood transfusion is not yet understood, and the clinical relevance is a controversial topic of discussion. METHODS The effect of blood transfusions on the capacity of the host's immunocompetent cells to react to mitogenic stimulation was evaluated. Patients undergoing hip replacement surgery received either allogeneic (n = 13) or autologous (n = 14) buffy coat-depleted red cells or plasma. Patients' blood samples taken before and on Days 1 and 5 after surgery were stimulated in a whole-blood assay. The release of interleukin 2, soluble interleukin 2 receptor, interleukin 6, tumor necrosis factor alpha, interferon alpha 2, and interferon gamma was assessed by enzyme-linked immunosorbent assay. In addition, the white cell counts and frequencies of the lymphocyte subsets CD4+, CD8+, and natural killer cells were analyzed. RESULTS For both groups, decreased levels of interleukin 2 and interferon-gamma were detected postoperatively, whereas the values for soluble interleukin 2 receptor and tumor necrosis factor alpha showed no significant change. Interferon alpha 2 was decreased on Day 1, but returned to normal by Day 5. Interleukin 6 increased during the time of observation. There were no significant differences between the two groups in cytokine production and lymphocyte-subset analysis that could be attributed to the transfusion of allogeneic blood. CONCLUSIONS The transfusion of buffy coat-depleted red cells showed no immediate suppressive effect on the immune function of the host's peripheral blood cells.