The susceptibility of 310 mainly clinically isolated Gram-negative nonfermenters to aztreonam, carumonam, ciprofloxacin, ofloxacin and fleroxacin was determined by agar dilution. All 3 quinolones were very active, with MIC90 ranges (micrograms/ml) against all organisms ranging between 0.25 and 8 for ciprofloxacin, and 0.25 and 16.0 for ofloxacin and fleroxacin, respectively. Aztreonam and carumonam showed less activity than the quinolones on a weight-for-weight basis, with MIC90 values ranging from 4 to 32 for both drugs; only Pseudomonas aeruginosa and Acinetobacter calcoaceticus biotypes haemolyticus and alcaligenes were uniformly susceptible to both monobactams. The broad-spectrum activity of the 3 quinolones suggests potential use in therapy of infections caused by nonfermenters; monobactams should be reserved for infections caused by P. aeruginosa and possibly Acinetobacter spp.