The monthly variation in geometric mean MICs (GMICs) of amikacin, tobramycin, and gentamicin for Escherichia coli and Pseudomonas aeruginosa laboratory controls and clinical isolates was followed for 30 months. For 19 months, MICs were determined by using Micro-Media system (MMS; Micro-Media Systems, Inc., Potomac, Md.) microdilution panels, and for the other 11 months, MicroScan (MS; American Scientific Products, McGaw Park, Ill.) MIC test panels were used. The aminoglycoside GMICs for control E. coli and P. aeruginosa were significantly lower with the MS system than with the MMS system. A significant correlation was observed between the GMICs for controls and clinical isolates more frequently with the MMS system than with the MS system. Differences are believed to be related to the criteria used in the selection of quality control strains.