This three-center collaborative study was conducted to evaluate samples of the Microbial Profile System (MPS) antimicrobial microdilution panels [previously produced by Minnesota Mining & Manufacturing Co., (3M Co.), St. Paul, Minn. and currently produced by Flow Laboratories, Inc., Rockville, Md.). This was a three-phase study. In phase I, the inter- and intralaboratory agreement was determined by using strains with selected ranges of susceptibility. The MPS and reference microdilution minimum inhibitory concentrations were within acceptable variation, +/- 1 dilution for 97.7% for the MPS and 98.8% for the reference microdilution panels for the intralaboratory comparisons. The percentage of strains with minimum inhibitory concentrations in the acceptable range for the interlaboratory variation was 96.2% for the MPS and 96.0% for the reference microdilution panels. The phase II studies used strains with known resistance mechanisms. The percent agreement with these strains was: Enterobacteriaceae, 94.5%; nonenteric gram-negative rods, 95.4%; staphylococci, 92.3%; and streptococci, 96.6%. The overall agreement within acceptable limits was 94.7% with these strains. When testing 359 clinical isolates, the frequency of strains within the acceptable range of agreement between the two methods was 97.3%. The MPS panels gave results in each of the three study phases equivalent to those obtained with the reference microdilution panels.