The concentration of ampicillin in serum and bile has been determined in 10 patients after oral administration of equivalent doses of ampicillin (500 mg) and pivampicillin (700 mg). Ampicillin produced maximum concentrations in bile twice as high as in serum, and, after pivampicillin, peak levels of ampicillin occurred earlier and were approximately four times greater than after ampicillin. In 11 other patients, ampicillin was determined simultaneously in peripheral and portal blood, collected from an umbilical vein catheter, following a dose of pivampicillin or an intravenous injection of ampicillin. The peak concentration of ampicillin after an oral dose was twice as high in portal blood as in peripheral blood. There appears to be passive transfer of ampicillin from blood to bile.