The pharmacokinetic parameters of two new ureido-penicillins (azlocillin and mezlocillin) were determined in 12 healthy subjects after a half-hour continuous infusion of 5,000 mg. The agar diffusion test (test strain Bacillus subtilis ATCC 6633) was used for the microbiological assays. The mean azlocillin serum concentration after the half-hour infusion was 431.0 +/- 75.0 microgram/ml; after eight hours it had fallen to a mean value of 4.7 +/- 2.6 microngram/ml. The mean elimination half-life was 77.5 +/- 10.4 minutes, and the relative distribution volume was 19.4 +/- 1.9% of the bodyweight. At the end of the infusion, mezlocillin showed a mean serum concentration of 426.0 +/- 61.0 microgram/ml and after eight hours an average of 1.1 +/-0.9 microgram/ml; the half-life was shorter (56.9 +/- 9.9 minutes) and the distribution volume lower (14.8 +/- 3.1%) than that of azlocillin. The renal clearance values measured in three subjects during a four-hour continuous infusion were: azlocillin 111.6 ml/min/1.73 m2, mezlocillin 121.5 ml/min/1.73 m2. The kinetic behaviour of the two ureido-penicillins was essentially very similar to that of ampicillin and carbenicillin, 38 patients with bronchopneumonia, cholangitis or urinary tract infections, which in some instances were severe, were treated for an average of 10 days with an average daily dosage of 3X4.0 g azlocillin or 3X5.0 g mezlocillin. 30 patients showed clinical improvement, and in 17 of these the pathogen was eliminated. These therapeutic results appear more favourable than those obtained with the newer aminoglycoside antibiotics (amikacin, sisomicin); in particular the drug was well tolerated.