Pristinamycin IA is a cyclo-peptidic macrolactone antibiotic belonging to the streptogramin family. In the present work, the interaction of pristinamycin IA with the multidrug transporter P-glycoprotein was investigated in the differentiated human intestinal epithelial cell line Caco-2. Pristinamycin IA specifically inhibited the efflux of the P-glycoprotein substrate [3H]vinblastine, thus increasing the cellular accumulation of the drug. Pristinamycin IA also reduced by 70% the basolateral to apical secretion of [3H]vinblastine across Caco-2 cell monolayers. The cellular accumulation of [14C]pristinamycin IA was very low and was increased by P-glycoprotein inhibitors (verapamil, chlorpromazine and reserpine). The basolateral to apical transport of [14C]pristinamycin IA was 100-fold higher than apical to basolateral passage. This polarized transport was inhibited by verapamil and by ATP depletion. The results suggest that pristinamycin IA is a substrate for the P-glycoprotein, a finding which may have important consequences for the pharmacokinetics of this drug.