From July 1974 through June 1976, a number of isolates of Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa from the Burn Center exhibited a shift to smaller zone diameters with gentamicin than did isolates from the general hospital population. Although many had zone diameters >/=13 mm and would have been considered susceptible by this breakpoint, they were found to have minimal inhibitory concentrations (MICs) of >/=8 mug of gentamicin per ml by agar dilution testing. Zone diameters and MICs of gentamicin, tobramycin, and amikacin were subsequently compared for 168 isolates from both the Burn Center and general hospital. The results revealed many isolates that fell into presently used gentamicin- and tobramycin-"susceptible" categories by disk diffusion tests but were resistant by MIC. The data indicated that criteria for gentamicin disk diffusion testing should include an intermediate or indeterminate category, and that the limits of the intermediate category for tobramycin and amikacin should be expanded.