Lomefloxacin (SC47111, Searle) is a difluorinated quinolone with a comparatively long half-life and high serum concentration. This agent has good in vitro activity against Enterobacteriaceae (MIC90 = less than 2 micrograms/ml) and staphylococci (MIC90 = less than 2 micrograms/ml); the activity against Pseudomonas spp. and Pseudomonas aeruginosa is moderate to poor. On a weight basis, lomefloxacin is less active than ciprofloxacin; however, based on the ratio of the serum concentration to MIC, the activity of lomefloxacin is nearly equivalent to ciprofloxacin against many bacteria, with the exception that ciprofloxacin has good activity against most pseudomonads. Also, lomefloxacin was active against a variety of bacteria that were resistant to aminoglycosides and/or third-generation cephalosporins. A majority of strains of the Mycobacterium avium complex, isolated from AIDS patients with disseminated disease, were found to be resistant to lomefloxacin (MIC90 = greater than 8 micrograms/ml).