In vitro and in vivo studies were conducted to investigate the drug interaction between a new quinolone antimicrobial, DU-6859a, and theophylline (TP). The effect of DU-6859a on TP metabolism was evaluated in vitro by measuring the rate of TP metabolite formation by using human liver microsomes. DU-6859a inhibited the metabolism of TP, especially the formation of 1-methylxanthine, in vitro, but to a lesser extent than other drugs that are known to interact with TP. TP was administered alone (200 mg twice a day [b.i.d.] for 9 days) or in combination with DU-6859a (50 or 100 mg b.i.d. for 5 days) to six healthy subjects. DU-6859a administered at a dose of 50 mg resulted in no changes in serum TP concentrations, and slight increases in serum TP concentrations were observed at a dose of 100 mg. Moreover, the administration of 100 mg of DU-6859a resulted in decreases in all urinary TP metabolites, with significant differences. It appears that although DU-6859a has a weak inhibitory effect on TP metabolism in vitro, its concomitant use with TP at clinical dosage levels does not cause any adverse effects, showing only a slight increase in blood TP concentrations and a decrease in urinary metabolites.