In a cross-over study, the oral absorption following equimolar doses of ampicillin corresponding to 16.7mg/kg administered as pivampicillin (‘Pondocillin’) suspension and ampicillin (‘Doktacillin’) suspension respectively, was investigated in 11 children aged from 8 months to 4.5 years. Pivampicillin produced a mean peak serum concentration of 10.7&mgr;g ampicillin/ml compared with 5.8&mgr;g/ml after ampicillin. The mean area under the serum concentration-time curves was 29.2&mgr;g/ml · h after pivampicillin and 16.7&mgr;g/ml · h after ampicillin, reflecting the superior bioavailability of the ester form.In 4 of the children aged between 8 and 12 months, the serum levels after pivampicillin were consistently lower than those recorded in the remaining 7 children beyond the age of 1 year. The mechanism and therapeutic implications of this finding require further study.