Administration of antibiotics in cases of open heart surgery under conditions of extracorporeal circulation requires careful management with regard to prophylaxis against infection as well as possible renal failure.In this study, the dose of semi-synthetic penicillin SB-PC was calculated from the sum of the assessed circulating blood volume, based on body weight and the priming volume. As a result of a study on the concentration in the circulating blood, the antibiotics was administered via drip infusion, at a constant rate throughout the operation, under extracorporeal circulation, in order to obtain a stable concentration.