On the absorption and metabolism of 35S-ampicillin.

An aqueous solution of 35S-ampicillin and polyethylene glycol (PEG 4000, unabsorbable marker) was administered orally to 6 healthy subjects with gastrointestinal tubes. The minimal absorption of radioactive material, as shown by urinary excretion of label, differed markedly between subjects and averaged 44% (25-67). As the mean cumulative uptake in the proximal jejunum was 31% (24-45), it appeared that some of the radioactivity had been taken up in more distal parts of the gastrointestinal tract. Chromatographic fractionation of gastrointestinal aspirates indicated that more than 95% of the label usually remained attached to the parent compound. The corresponding figure for radioactive material recovered from urine was about 80-85%. A similar degree of decomposition was also found for label excreted in the urine of an additional subject who received 35S-ampicillin intravenously. In this experiment the urine contained 87% of the administered radioactivity.