The in-vitro activity of lomefloxacin (SC 47111, NY-198) was investigated by the determination of MICs in agar and in broth, of MBCs in broth, of killing curves and of the duration of the post-antibiotic effect. MICs measured in broth and in agar were almost identical. Lomefloxacin was two- to eight-fold less active against Gram-positive bacteria than ofloxacin. Its activity against Staphylococcus aureus was independent of resistance to penicillin and oxacillin. The activity of lomefloxacin, ofloxacin and pefloxacin was poor against JK corynebacteria. Enterobacteriaceae, Aeromonas spp., Haemophilus, influenzae, Neisseriaceae and Campylobacter jejuni were highly susceptible to the three quinolones investigated. Non-fermenting Gram-negative bacilli were less susceptible. MBCs were within one dilution of the corresponding MICs. The killing rate was very high against Gram-negative bacilli (2.5-4.0 log cfu/ml reduction in 2 h) whereas it was low against Gram-positive bacteria (0.5-1.0 log cfu/ml reduction in 2 h). Emergence of resistance was not observed. The duration of the post-antibiotic effect with Gram-negative bacilli depended on the strain and species (median: 0.9-1.5 h). The post-antibiotic effect was insignificant with Staph. aureus.