A new semisynthetic ureidopenicillin (Bay K 4999) demonstrates favorably in vitro antibacterial efficacy against human-pathogenic gram-negative rods in comparison to mezlocillin and azlocillin. A comparative pharmacokinetic study was done with 10 test subjects after 30 min intravenous infusion of 4.0 g of Bay K and mezlocillin, respectively. The serum concentration course during a period of 10 h showed an open three-compartment model for both antibiotics. The urine recovery of Bay K 4999 during 24 h was only 32.6 ± 4.3% of the applied dose. In three test subjects with normal renal function, the average renal clearance of Bay K was 61.0, the total serum clearance was 409.2 ml/min/1.73 m2. 31 patients were treated with a daily dose of 3 × 1.0–2.0 g Bay K for severe bronchopulmonary, UTI and cholangiogenic infections. The therapeutic results were good; the relatively high number of side effects should be further investigated in animal studies and require more clinical experie