425 randomly selected, fresh clinical isolates were tested for susceptibility to norfloxacin and other orally absorbed agents, i.e. amoxicillin, ampicillin, carbenicillin (available commercially as the indanyl ester), cefaclor, cinoxacin, erythromycin, nalidixic acid, penicillin G, tetracycline, trimethoprim and co-trimoxazole. The results have shown norfloxacin to be the most potent agent in vitro against representative members of the family Enterobacteriaceae, Staphylococcus aureus, Pseudomanas aeruginosa, Acinetobacter spp., Neisseria gonorrhoeae and Haemophilus influenzae. Ninety percent of the isolates in these groups of bacteria were inhibited by less than 1 mg/l, 2 mg/l, 8 mg/l, 32 mg/l, 0.06 mg/l and 0.25 mg/l of norfloxacin, respectively. Although norfloxacin inhibited most streptococci and Ureaplasma at a concentration of 8 mg/l or less, penicillin G proved to be the most active against Streptococcus pygenes and Streptococcus pneumoniae; trimethoprim was the most active against Streptococcus faecalis, and tetracycline the most active against Ureaplasma.