The bactericidal activity of imipenem (IM), ticarcillin (TIC), ceftazidime (CEZ), amikacin (AMK) or tobramycin (TOB), ciprofloxacin (CIP), was compared on 6 P. aeruginosa: 1 ticarcillin, susceptible strain (TICs), 5 constitutive beta-lactamases producing strains (PSE, TEM, OXA1, OXA2, cephalosporinase (CEP). The time-kill-curves method was performed. Bacteria were incubated with antibiotics at M.I.C. X2 and at concentrations obtained in vivo with usual therapeutic doses: IM 4-8 mg/l, TIC 64-128, CEZ 4-32, TOB 2-8, AMK 4-16, CIP 1-4. The bactericidal activity of IM was independent of the concentration. A 5 Log10 reduction in viability was observed at 3 h for strains TICs, TEM and PSE, 3 Log10 for strains OXA and CEP. This bactericidal activity was also observed in a non growing system. The bactericidal activity of TIC (strain TICs) and CEZ was slower and weaker: 1-2 Log10 at 6 h, 2-3 Log10 at 24 h. It is not observed in a non growing system. On the other hand, the bactericidal activity of aminoglycosides and ciprofloxacin was rapid: 3-4 Log10 at 3 h, 5 Log10 at 6 h for all strains in growing and non growing systems. IM is the only beta-lactam antibiotic bactericidal on growing and non growing cells of P. aeruginosa like aminoglycosides and new quinolones.