This study was performed to determine the absolute PMNL count as well as phagocytic functions in adolescents who had undergone CPB; a radioactive method was used for the first time in this field. Although CPB causes a decrease in the absolute PMNL count when the subject is off bypass, this value exceeded the baseline level within three days. A day prior to surgery PMNL phagocytosis was found to be normal and was unaffected within the first minutes on bypass. Whereas, PMNL phagocytosis decreased significantly off bypass. However, the decrease was transient and returned to normal within three days. Another interesting finding obtained as a result of this study was that in the cases in whom the clinical infection picture was seen, the patient had a longer duration on bypass and a higher inhibition of leukocyte phagocytosis. The correlation between the duration on bypass and inhibition of leukocyte phagocytosis could be shown as a trend. Thus it may be concluded that the longer the on bypass period the higher the inhibition of leukocytic phagocytosis and the higher the infection rate in the postoperative period.