The MICs of ceftiofur and other antimicrobial agents, tested for comparison, for 515 bacterial isolates of pigs from the United States, Canada, and Denmark with various diseases were compared. The organisms tested included Actinobacillus pleuropneumoniae, Escherichia coli, Pasteurella multocida, Salmonella choleraesuis, Salmonella typhimurium, Streptococcus suis, Streptococcus dysgalactiae subsp. equisimilis, Streptococcus equi subsp. equi, and Streptococcus equi subsp. zooepidemicus. In addition to ceftiofur, the following antimicrobial agents or combinations were tested: enroflaxacin, ampicillin, sulfamethazine, trimethoprim-sulfadiazine (1:19), erythromycin, lincomycin, spectinomycin, lincomycin-spectinomycin (1:8), tilmicosin, and tetracycline. Tilmicosin was only tested against the U.S. isolates. Overall, ceftiofur and enrofloxacin were the most active antimicrobial agents tested against all isolates, with MICs inhibiting 90% of isolates tested (MIC(90)s) of less than or equal to 2.0 and less than or equal to 1.0 mu g/ml, respectively. Erythromycin, sulfamethazine, spectinomycin, and lincomycin demonstrated limited activity against all of the organisms tested, with MIC(90)s of greater than or equal to 8.0, greater than or equal to 256.0, greater than or equal to 32.0, and greater than or equal to 16.0 mu g/ml, respectively. Trimethoprim-sulfadiazine was active against isolates of A. pleuropneumoniae, S. choleraesuis, S. typhimurium, P. multocida, S. equi, and S. suis (MIC(90)s, less than or equal to 0.5 mu g/ml) but was less active against the E. coli strains tested (MIG(90), >16.0 mu g/ml). Ampicillin was active against the P. multocida, S. suis, and S. equi isolates tested (MIC(90)s, 0.5, 0.06, and 0.06 mu g/ml, respectively) and was moderately active against S. typhimurium (MIC(90)s, 2.0 mu g/ml). However, this antimicrobial agent was much less active when it was tested against A. pleuropneumoniae, S. cholerae-suis, and E. coli (MIC(90)s, 16.0, >32.0, and >32.0 mu g/ml, respectively). Against the U.S. isolates of A. pleuropneumoniae and P. multocida, tilmicosin was moderately active (MIC(90)s, 4.0 and 8.0 mu g/ml, respectively). However, this compound was not active against the remaining U.S. isolates (MIC(90)s, >64.0 mu g/ml), Differences in the MICs from one country to another were not detected with enrofloxacin, ceftiofur, or lincomycin for the strains tested, but variations in the MICs of the remaining antimicrobial agents were observed.