In a randomized cross-over study 10 healthy male volunteers received a single 1 g dose of 6-[R]-2-3-methylsulfonyl -2- oxo-imidazolisine -1- carboxamido) -2- phenyl-acetamido]-penicillanic acid sodium salt (mezlocillin) and of ampicillin, either intravenously or intramuscularly. Following the intravenous loading dose, mean peak serum levels of 101 micrograms/ml for mezlocillin and 91.5 micrograms/ml for ampicillin were recorded. The ultimate half-life t 1/2 of mezlocillin (46.4 min) was slightly shorter than that of ampicillin (52.4 min). Similarly, the total volume of distribution of mezlocillin (24.1 l) was slightly inferior to that of ampicillin (29.4 l). The proportions of the dose administered recovered in the urine of 24 hours were 50.4 % for mezlocillin and 69.9 % for ampicillin. The total clearances of the two antibiotics were not significantly different, but the renal clearance of mezlocillin (186.6 ml/min/1.73 m2) was significantly lower than that of ampicillin (309.5 ml/min/1.73 m2). Following intramuscular injection, the mean peak serum levels obtained were 15.6 microgram/ml with mezlocillin and 15.1 micrograms/ml with ampicillin. The half-lives of the antibiotics were 50.0 min and 57.2 min respectively. The bioavailable fractions of mezlocillin and ampicillin, as measured from the areas under the serum concentration curves wer 63 % and 75 % respectively of the values determined after intravenous injection. The oral administration of 1 g probenecid one hour before an intramuscular injection of mezlocillin increased the peak serum level and area under the curve by 65 % and decreased the total clearance, renal clearance and apparent volume of distribution by 38 %, 52 %, and 35 % respectively. However, the ultimate half-life was not significantly altered (50.5 min without, and 52.0 min with probenecid).