AR-C155858 is a novel, selective and potent inhibitor of monocarboxylate transporters MCT1 and MCT2 with Kis of 2.3 nM and 10 nM, respectively. It binds to an intracellular site involving transmembrane helices 7-10. Inhibitor titrations of L-lactate transport by MCT1 in rat erythrocytes were used to determine the Ki and number of AR-C155858-binding sites (Et) on MCT1 and the turnover number of the transporter (kcat). Derived values were 2.3+/-1.4 nM, 1.29+/-0.09 nmol per ml of packed cells and 12.2+/-1.1 s-1 respectively.