The activity of amikacin, gentamicin and kanamycin was tested in vitro against clinical isolates of Pseudomonas aeruginosa. Concentrations of the antibiotics in serum and in saline solution were prepared according to serum levels produced in volunteers 15 minutes, 1,2, and 6 hours after a single intramuscular injection of 500 mg amikacin, 80 mg gentamicin and 500 mg kanamycin. Following isolation of the Pseudomonas strains in cultures, they were incubated and seeded in Mueller-Hinton broth, then 10(-7) dilutions of the organisms were kept in contact with the prepared antibiotic solutions in serum and in saline solution for three hours, the approximate half-life of the antibiotics in serum. Amikacin was active at concentrations seen six hours post-dose, inhibiting growth in a total of 72-5% of seeded plates. Gentamicin was active for only two hours and inhibited growth in 2-5% of the plates. Kanamycin showed no antipseudomonal activity.