Azlocillin (Securopen) and mezlocillin (Baypen) were given to 3 healthy subjects as intravenous infusion. The dose of 4 g was administered to each person within 5, 15, and 30 min in a randomized crossover design. Using HPLC the unchanged penicillin antibiotics were determined quantitatively, their metabolites were assessed qualitatively. The same specimens were also studied by means of a bioassay (agar diffusion technique). Both methods yielded similar serum and urine concentrations besides the urinary excretion of azlocillin. Here the bioassay measured higher amounts indicating an antibacterially active metabolite being excreted in the urine. No dependence upon infusion time was found. Since both drugs were tested with the same dosis in the same subjects, their pharmacokinetic parameters could be compared: mezlocillin, being more lipophilic than azlocillin, showed a higher volume of distribution and therefore lower serum concentrations. Renal clearance was the same for both drugs, but mezlocillin was excreted to a smaller extent in the urine. Higher total clearance and shorter elimination half-life of mezlocillin indicate a greater extrarenal elimination. The results suggest fast application of both penicillins. There is no pharmacokinetic reason for a prolongation of infusion times.