The in-vitro antibacterial activity of nalidixic acid and the 4-quinolones, ciprofloxacin, norfloxacin, enoxacin, ofloxacin, Pefloxacin, A-56619, A-56620 and CI-934 was assessed by determination of MICs. The 4-quinolones were all highly active against most isolates of Enterobacteriaceae, including nalidixic acid-resistant strains. Ciprofloxacin (MICs 0·002-2 mg/1) was the most active and A-56619 (MICs 0·008-32 mg/1) was the least active. A-56619, A-56620, ofloxacin, ciprofloxacin and CI-934 were highly active against Acinetobacter strains, Pefloxacin and enoxacin were slightly less active, and a few strains were resistant to norfloxacin. All the compounds, including nalidixic acid, were active against Aeromonas strains (MICs 0·001-0·12 mg/1). Ciprofloxacin (MICs 0·06-1 mg/1) was the most active compound against Pseudomonas aeruginosa; A-56619 and CI-934 (MICs 1-16 mg/1) were the least active against this species.. All the compounds were highly active against Haemophilus influenzae, Branhamella catarrhalis and Neisseria gonorrhoeae but the activity of all the compounds was poor against most isolates of Gardnerella vaginalis. All the 4-quinolones were active against staphylococci and CI-934 (MICs 0·03-0·25 mg/1) was the most active. CI-934 (MICs 0·06-2 mg/1) was also the most active compound against all streptococci. Most streptococci were sensitive also to ciprofloxacin (MICs 0·25-4 mg/1) but there were many isolates resistant to the other 4-quinolones. Against the anaerobic bacteria CI-934 was again the most active compound, particularly against the Gram-positive anaerobic cocci. Pefloxacin, enoxacin and norfloxacin had poor activity against most anaerobes. Ofloxacin, ciprofloxacin, A-56619 and A-56620 had good to moderate activity against all species of anaerobes except the Bacteroides fragilis group, against which none of the compounds was very active.