Of 97 well-defined strains of Pseudomonas, isolated from sputum of patients with cystic fibrosis (CF), the minimum inhibitory concentration (MIC) of several antibiotics was determined with a broth dilution method. The majority of the strains were resistant to ampicillin and cephalothin, moderately susceptible to carbenicillin (70% to 100 microgram/ml) and highly susceptible to piperacillin (100% to 25 microgram/ml, 88% to 6.25 microgram and 60% to 3.12 microgram/ml). If the pharmacological properties of piperacillin are comparable with those of carbenicillin, it can be expected that the sputum level of this drug will be adequate to treat Pseudomonas pulmonary infections. At the lowest concentration tested (0.78 microgram/ml) 3% of the strains were susceptible to kanamycin, 85,5% to amikacin, 95% to gentamicin, 98% of tobramycin, and 80% to colimycin. With regard to clinically attainable concentrations, 98.9% of the strains were susceptible to gentamicin and tobramycin 97.9% to amikacin, 96.9% to colimycin, 88.6% to piperacillin, 38% to carbenicillin, 25.7% to kanamycin, 12.3% to ampicillin, and 1% to cephalothin.