Microemboli may impair cognitive function in patients undergoing heart surgery. Prebypass filtration has been shown to reduce particle load in the cardiopulmonary bypass (CPB) priming fluid. This study was performed to detect the embolic load of CPB priming fluid, to determine the efficacy of a 0.2 microm prebypass filter (PBF) in reducing emboli in the range of 0.1-5 microm and to provide guidelines for the handling of the device. A total of 12 CPB circuits were tested in two groups, using a laser light scattering particle counter, sensitive to microemboli in the range of 0.1-5 microm. In control group A, priming fluid before administration to the CPB circuit was analyzed. Group B circuits contained microporous membrane oxygenators (N = 5); group C consisted of CPB circuits with excluded membrane oxygenators (N = 7). When group A was compared to groups B and C, significantly more microemboli were found in the categories 0.2 microm, 0.5 microm, 0.8 microm for both groups B and C (p < .05). Group C circuits had higher microemboli counts in the categories 1.5 microm and 3 microm (p < .05) when compared to group B. Microemboli bigger than 0.2 microm could be eliminated after 2 min of prebypass filtration with a CPB flow of 5 L/min. The number of microemboli smaller than 0.2 microm was reduced substantially. Small microemboli with a size of 0.1 microm originate mainly from the priming solution. Microemboli in the range of 0.2 microm, 0.5 microm, and 0.8 microm originate mainly from the CBP circuit. In circuits with bypassed membrane oxygenators, a higher microemboli count in the range of 1.5 microm and 3 microm may be explained by a possible filtering capacity of membrane oxygenators. The 0.2 microm PBF is an effective tool to reduce the particle load in the CPB priming fluid.