Equimolar doses of bacampicillin, which is rapidly converted to ampicillin in the body by hydrolysis, and ampicillin were administered orally, in the case of ampicillin also by intravenous injection, to 10 healthy subjects (cross-over study). Comparison of the areas under the serum concentrations curves after intravenous and oral administration showed that bacampillin was absorbed to 95% and orally given ampicillin to 35%. The mean peak serum levels (Cmax) after 0.8 g of oral bacampicillin were higher (15.9 microgram/ml) and appeared earlier (tmax 60 min) than after 0.556 g of oral ampicillin (3.2 microgram/ml, tmax 150 min). One and three hours after oral administration skin blister fluid contained four times more ampicillin after doses of bacampicillin than after oral ampicillin. One hour after intravenous injection of ampicillin the skin blister concentrations were 20 times higher than after oral administration of this antibiotic and three to four times higher than after oral administration of bacampicillin. The levels in saliva and tears were also determined and showed similar relationships. Since higher peaks serum levels resulted in higher and longer lasting concentrations in the extravascular space, bacampicillin is to be preferred for oral therapy.