Multilaboratory studies have been carried out to establish quality control limits for broth microdilution susceptibility tests with 39 different antimicrobial agents and four different control strains. For each drug-microorganism combination, 100 MIC values were recorded (20 from each of five laboratories). An acceptable range of MICs was defined as values that included greater than 95% of those 100 MICs. In a series of consecutive tests, less than or equal to 5% of the MIC values might be expected to be one doubling dilution interval above or below the limits defined in that way: at least 95% of those MIC determinations should be within the narrow control limits. The 39 antimicrobial agents provided 111 drug-microorganism combinations for which usable quality control limits could be defined. The greater than 95% rule yielded a narrow range (including only two dilution steps) with 58 of the 111 control limits, and 53 control limits permitted a more traditional three-dilution step range of MICs.