Bactericidal activity of gentamicin, imipenem, ciprofloxacin and cefotaxime associated with sulbactam are studied on 36 beta lactamase CTX-1 (TEM-3) producing Klebsiella pneumoniae strains isolated of sputum samples and urines, among patients of intensive care units, in Rangueil hospital (Toulouse). Gentamicin, imipenem, cefotaxime associated with sulbactam have a bactericidal activity at serum levels, obtained with usual therapeutic doses. Ciprofloxacin has a bactericidal activity at concentrations obtained in urine and into lung tissue. Killing curve method was used to study bactericidal activity of these antibiotics alone and in combination, on a representative sample. Alone gentamicin showed the most rapid bactericidal activity (2 hours). The associations with gentamicin are also rapidly bactericidal. The associations with ciprofloxacin and imipenem, ciprofloxacin and cefotaxime/sulbactam are synergistic.