A wide variety of antibacterial compounds is rapidly oxidized by O-3 and hydroxyl radical ((OH)-O-center dot) during aqueous ozonation. Quantitative microbiological assays have been developed here or adapted from existing methods and utilized to measure the resulting changes in antibacterial potencies during O-3 and (OH)-O-center dot treatment of 13 antibacterial molecules (roxithromycin, azithromycin, tylosin, ciprofloxacin, enrofloxacin, penicillin G, cephalexin, sulfamethoxazole,trimethoprim, lincomycin, tetracycline, vancomycin, and amikacin) from 9 structural classes (macrolides, fluoroquinolones, beta-lactams, sulfonamides, dihydrofolate reductase inhibitors, lincosamides, tetracyclines, glycopeptides, and aminoglycosides), as well as the biocide triclosan. Potency measurements were determined from dose-response relationships obtained by exposing Escherichia coli or Bacillus subtilis reference strains to treated samples of each antibacterial compound via broth micro- or macrodilution assays and related to the measured residual concentrations of parent antibacterial in each sample. Data obtained from these experiments show that O-3 and (OH)-O-center dot reactions lead in nearly all cases to stoichiometric elimination of antibacterial activity (i.e., loss of 1 mole equivalent of potency per mole of parent compound consumed). The beta-lactams penicillin G (PG) and cephalexin (CP) represent the only clear exceptions, as bioassay measurements indicate that biologically active products may be formed in the reactions of these two compounds with both O-3 and (OH)-O-center dot. The active product(s) generated in the direct reaction of O-3 with PG appear(s) to be recalcitrant to further transformation by O-3, though any biologically active products formed in the reactions of CP with O-3, or of either PG or CP with (OH)-O-center dot, are apparently deactivated by further reactions with O-3 or (OH)-O-center dot, respectively. Thus, with few exceptions, it can be expected that municipal wastewater ozonation will generally yield sufficient structural modification of antibacterial molecules to eliminate their antibacterial activities, whether oxidation results from selective reactions with O-3 or from relatively nonselective reactions with incidentally produced (OH)-O-center dot.