Ponsinomycin is a new macrolide. Its pharmacokinetics is characterized by an extensive metabolism. Following oral dosing in human, unchanged ponsinomycin is not found in blood, but 3 main metabolites: Mb12, Mb6 and Mb9a can be assayed. Twelve young healthy volunteers received ponsinomycin orally, at doses equal to 400, 800 and 1,200 mg on 3 separate occasions, according to a cross-over design. Metabolites Mb12, Mb6 and Mb9a were measured in plasma by HPLC. The pharmacokinetics of Mb12 was linear in the range of doses administered. A dose-dependent effect leading to a reduction of the apparent elimination rate when dose increased was observed with Mb9a and more importantly with Mb6.