The kinetics of mezlocillin, a semisynthetic acylureido penicillin, more active than carbenicillin against many gram-negative bacteria, were compared with those of carbenicillin. Following an intravenous infusion of 4 gm in 5 min to 8 normal men there was an average serum level of 294 µg/ml for mezlocillin and 365 µg/ml for carbenicillin. The t½ for mezlocillin was 47 min and that for carbenicillin was 70 min. The apparent volume of distribution was 13.4 L for mezlocillin and 14.4 Lfor carbenicillin. The mean urinary recovery of mezlocillin was 72% and that for carbenicillin was 92 %. Constant infusion of 5 mg of mezlocillin over 2 hr gave steady-state levels of 234 µg/ml. Half-life, apparent volume of distribution, and serum and renal clearance of mezlocillin after constant infusion were in the same range as those after rapid infusion.