The United States Food and Drug Administration recently recommended that the antimicrobial product package insert (PPI) subsection on microbiology be annually validated with regard to the compound's spectrum and potency. To address this request, a nine-laboratory trial was organized to test (two methods) lomefloxacin, a newer fluoroquinolone, and nine comparison drugs against PPI-listed pathogens (1934 strains). A broad geographic sampling (nine medical centers) was achieved, and lomefloxacin was determined to be active [minimum inhibitory concentration (MICs), < or = 2 micrograms/ml for > or = 90% of strains] for all PPI-listed species except Pseudomonas aeruginosa, Citrobacter freundii, and Providencia rettgeri (42%-87% inhibited). Comparison fluoroquinolones also had a similarly compromised spectrum of activity against these species. Additional organism species, including Neisseria gonorrhoeae, N. meningitidis, Salmonella enteriditis, and Shigella species, should be added to the lomefloxacin PPI (MIC90s, 0.03-0.25 microgram/ml) following data generated in this study. These in vitro results indicate that lomefloxacin remains active against the vast majority of clinically "indicated" species, and that it has a spectrum compatible with other marketed fluoroquinolones for these tested pathogens, monitored in 1994.