The minimal inhibitory concentrations (MICs) of twelve 4-quinolone antimicrobials were determined for 100 isolates of Haemophilus influenzae (including 30 beta-lactamase producing strains) and 100 isolates of Streptococcus pneumoniae. MICs were determined using an agar dilution technique in Mueller-Hinton agar supplemented with 10% lysed horse blood. The inoculum used was approximately 10(4) colony-forming units, contained in 10 microliters of Mueller-Hinton broth, which was applied to the agar plates using a multipoint inoculator. Following inoculation, plates were incubated at 37 degrees C for 18 h in an atmosphere enriched to 10% carbon dioxide. The MIC of each antimicrobial for each isolate examined was determined as the lowest concentration of the antimicrobial which completely inhibited growth of the inoculum. The minimum concentrations required to inhibit the growth of 50% (MIC50) and 90% (MIC90) of the organisms examined were also determined. The more recently synthesised 4-quinolones showed considerably greater activity than nalidixic acid and pipemidic acid against clinical isolates of Haemophilus influenzae and Streptococcus pneumoniae. There was no apparent difference between the MICs observed for beta-lactamase producing and non-beta-lactamase producing strains of Haemophilus influenzae. Ciprofloxacin was the most active 4-quinolone examined (MIC90 for Haemophilus influenzae 0.008 microgram/ml; Streptococcus pneumoniae 2 micrograms/ml). Clinical studies on a possible role for some of the more recently synthesised 4-quinolones in the management of patients with respiratory infection are indicated.