Cardiac surgery using cardiopulmonary bypass (CPB) causes a systemic inflammatory response. Additionally, an impairment of the responsiveness of peripheral blood mononuclear cells (PBMC) to further immunological stimuli has been observed. The aim of our present study was to evaluate the ability of antioxidant therapy with mannitol or haemofiltration during CPB to modulate this immunosuppression after CPB. Forty-five patients undergoing elective heart-surgery were prospectively enrolled and randomized into three groups (control, mannitol, haemofiltration). Blood samples were taken after induction of anaesthesia (T1), 20 min after CPB (T2) and 24 h post-operatively (T3). Expression density of the monocytic surface receptor CD14, HLA-DR expression and cytokine release (TNF-alpha and IL10) after lipopolysaccharide-stimulation were evaluated. At T2, the CD14(dim) cell population was maintained in both intervention groups while in the control group there was a decrease of this proinflammatory monocytic phenotype. No significant differences regarding HLA-DR expression or cytokine release could be demonstrated. This study shows that the suppression of the stimulated immune response after CPB can potentially be alleviated by mannitol or haemofiltration in an experimental in-vitro setting. In the light of data showing that this depression of the immune response might affect the post-operative course of patients, these results could have a potential clinical relevance.