The search for erythromycin derivatives with improved antibacterial and/or pharmacokinetic properties, has led to the synthesis of several new agents. Roxithromycin, an ether oxime derivative of erythromycin, is one of the more promising. The main differences between erythromycin and roxithromycin are their kinetics, roxithromycin giving higher serum concentrations than erythromycin at equimolar oral doses. Its elimination half-life is also longer, about 12 h compared to 2-3 h for erythromycin. As their tissue distributions and antibiotic profiles are similar, roxithromycin can be administered in lower daily doses and at less frequent intervals. A suitable dosage regimen for roxithromycin seems to be 150 mg every 12 h. From a pharmacokinetic point of view, daily dosing with roxithromycin would be equivalent to the administration of erythromycin every 6 h.