The minimal inhibitory concentration for tobramycin and gentamicin against 488 clinical isolates of Enterobacteriaceae, Staphylococcus aureus and Pseudomonas spp. were obtained on three different solid media. The results obtained on Antibiotic Sensitivity Medium AB Biodisk (PDM) were markedly different from the results obtained on Mueller-Hinton (MH) and Diagnostic Sensitivity Test Agar Oxoid (DST). It was shown that tobramycin was significantly disfavored relative to gentamicin by PDM agar compared to MH and DST agar.