The in vitro activities of the three aminoglycoside antibiotics, gentamicin, and tobramycin have been compared against 500 isolates of Enterobacteriaceae, Pseudomonas aeruginosa and Staphylococcus aureus. An agar dilution method was employed with three sensitivity media: Iso-Sensitest Agar, Mueller-Hinton Agar and PDM-Antibiotic Sensitivity Medium. All three aminoglycosides were highly active against S. aureus (MIC less than or equal to 0.5 mg/l) and the majority of Enterobacteriaceae (MIC90 approx. 1 mg/l). Tobramycin showed the highest activity against P. aeruginosa (MIC less than or equal to 1 mg/l). No major difference in measured MIC were found on the three media. Gentamicin and netilmicin were somewhat less active against P. aeruginosa on Mueller-Hinton Agar. MICs for tobramycin against Enterobacteriaceae were a little higher on PDM than on the other two agars. Our results show that measured MIC varied very little on the three sensitivity media. All media are suitable for routine use, provided that control strains are employed.