In nine consecutive hospitalized patients treated with either ampicillin or amoxycillin, studies on the pharmacokinetics of orally administered amoxycillin and bacampicillin gave a mean absorption of 73% (+/- 15%) of the dose for amoxycillin and 65% (+/- 17%) for bacampicillin. An analysis of variance showed that the difference between the two drugs has only borderline significance (P less than 0.10). These mean values are lower than those reported in the literature for healthy volunteers, and the variation between patients is also larger. The mean absorption rates found for the two drugs did not differ from those found in healthy volunteers, amoxycillin being absorbed significantly more slowly than bacampicillin (P less than 0.02), but there were also large differences between patients.