The influence of calcium and magnesium in free form on the sensitivity of Pseudomonas aeruginosa to aminoglycoside antibiotics was evaluated on 42 clinical isolates. When compared to the minimal inhibitory concentrations (MIC) measured in Mueller-Hinton broth - a medium almost devoid of Ca and Mg - the MIC obtained in the same medium, supplemented with calcium and magnesium to achieve physiological concentrations of the free divalent ions (50 micrograms/ml and 10 micrograms/ml respectively), increased by a mean factor of 8 for gentamicin. Consequently, the percentage of gentamicin resistant strains increased from 2 to 78% when tested in the supplemented medium containing physiological concentrations of free calcium and magnesium. Similar increases in MIC were observed for tobramycin and amikacin. These results cast doubt on the clinical relevance of antibiotic disc sensitivities performed on Mueller-Hinton agar by the Kirby-Bauer technique. Moreover, they could explain some of the therapeutic failures observed with Pseudomonas aeruginosa strains falsely reported as sensitive to aminoglycosidic antibiotics.