Using the membrane filtration method bactericidal activity of amikacin and gentamicin as a function of time and concentrations was determined. Amikacin was bactericidal against all Pseudomonas aeruginosa (n = 7) and Klebsiella pneumoniae strains (n = 8) tested. In 5 of the 7 P. aeruginosa and 3 of the 8 K. pneumoniae strains we saw a secondary regrowth at 24 h. There was no difference between amikacin and gentamicin. The experiments to determine the effect of concentration on bactericidal activity showed that higher concentrations lead to a faster kill rate.