Objective To investigate the effects of probiotics on the plasma diamine oxidase (DAO) activity and D-lactate, IL-6 and IL-10 levels in patients undergoing open heart surgery under cardiopulmonary bypass (CPB) , trying to elucidate the mechanism of protective effect of probiotics against CPB- induced injury to intestinal mucosal barrier. Methods Forty ASA Ⅱ - Ⅲ patients of both sexes aged 30-64 yr weighing 40-80 kg undergoing open heart operation under CPB were randomized into 2 groups ( n = 20 each) : control group (group C) and probiotics group (group Y) . Group Y received Jinshuangqi (Bifid Triple Viable containing bifido-bacterium, lacto-bacillus, streptococcus thermophiles) 4 pills 3 times a day for 7 days before operation. Venous blood samples were taken from CVP line before operation (T1 ), at 10 min after aortic unclamping (T2 ) and at the end of CPB (T3 ) and at 2, 6, 18 h (T4,5,6) after operation for determination of plasma DAO activity and D-lactate, IL-6 and IL-10 levels.Results Plasma DAO activity, D-lactate, IL-6 and IL-10 levels were significantly increased after CPB was started at T2-6 as compared with the baseline values at T1 in both groups. Plasma DAO activity and D-lactate level were significantly lower at T2-5 , the plasma IL-6 level was significantly lower and plasma IL-10 level higher at T4,5 in group Y than in group C. Conclusion Probiotics can protect intestinal mucosal barrier in patients undergoing open heart surgery under CPB and attenuate inflammatory response.