Micro-Media Systems (MMS) has developed a procedure by which microdilution trays can be filled with dilutions of antimicrobial agents, frozen, and distributed to clinical laboratories. The trays are prepared in various distribution centers throughout the United States to supply clinical laboratories in the vicinity of each center. For use, trays are removed from the freezer, allowed to thaw, and then inoculated with inocula prepared as for any other susceptibility test, using a convenient disposable inoculator (ca. 5 mul per well). A collaborative study was planned to evaluate microdilution trays prepared in three Micro-Media Systems distribution centers. Microdilution minimal inhibitory concentrations (MICs) were compared to standard tube dilution tests (the international collaborative study group method). With gram-positive cocci, the two techniques gave essentially equivalent results. With gram-negative bacilli, the microdilution MICs were generally one doubling dilution lower than the standard tube dilution MICs. Similar results were seen with microdilution trays prepared with a Cooke Dynatech MIC 2000. Inter- and intralaboratory reproducibility with the macro- and microdilution techniques were quite satisfactory, i.e., at least 96% of the end points were within a range of +/-1 log(2) dilution intervals.