Even though blood transfusion-associated immunomodulatory effects have been reported, the basic immune mechanism is still not understood. Data from studies on the clinical effects of allogeneic blood-induced immunosuppression are contradictory. However, there are indications that autologous blood transfusion is not immunologically neutral but has intrinsic immunomodulatory potential. Therefore we investigated in vivo different immunological mediators in 56 randomized patients of a study comparing autologous and allogeneic blood transfusion in colorectal cancer surgery. Soluble IL-2 receptor, which is an indicator of general immune activation and the following immunologic refractory phase, indicated immunosuppression was more elevated at the seventh postoperative day in patients with allogeneic transfusions (p = .013) and autologous transfusions (p = .0003). The immunologic determination of TNF-alpha showed a significant postoperative increase in patients with autologous transfusions only (p = .0031). However, postoperative increase of soluble TNF-receptors p55 and p75 was also significant in patients transfused with allogenic blood (p = .022; p = .0014). The response to tetanus toxoid vaccination, an indicator of humoral immunity, was higher in patients transfused with allogeneic rather than autologous blood (p = .082), whereas responses of patients with autologous transfusions were even lower than in nontransfused patients. The reciprocal was already found for cell-mediated immunity determined by epicutaneously tested delayed-type hypersensitivity-reactions. IL-10 levels, an indicator of cellular immunosuppression, were determined in 27 additional patients before operation, immediately postoperative, and at the seventh postoperative day. IL-10 was found elevated immediately postoperative in allogeneic (p = .011) and nontransfused patients only (p = .042). The data from this study substantiate recent findings of a different immunomodulatory potential of allogeneic and autologous blood transfusion. They furthermore support the hypothesis that autologous blood transfusion does not contain immunologically neutral effects of allogeneic blood, but itself exerts an immunomodulatory effect.