Plasma protein binding of a wide range of gyrase inhibitors in clinical practice or trials has been determined by ultrafiltration to determine structure-protein binding relationships. The protein binding was independent of overall lipophilicity. In particular, the "western" part of the "quinolone" skeleton, consisting of a heterocyclus at position 7 and varying substituents at position 8, strongly influences the extent of protein binding, indicating that this part interacts with the plasma protein. In contrast, substituents in position N1 do not show an effect on the protein binding in this series of compounds.