Lactate and pyruvate cross cell membranes via a monocarboxylate transporter (MCT) with well-defined properties but undefined molecular structure. We report the cloning of a cDNA encoding MCTl, a monocarboxylate transporter whose properties resemble those of the erythrocyte MCT, including proton symport, trans acceleration, and sensitivity to a-cyanocinammates. A Phe to Cys substitution in MCTl converts it to Mev, a mevalonate transporter. MCTl is abundant in erythrocytes, cardiac muscle, and basolateral intestlnal epithelium. In skeletal muscle it is restricted to mitochondria-rich myocytes. As sperm traverse the epididymis, MCTl switches from sperm to epithellal cells. MCTl is present at low levels in liver, suggesting another MCT in this tissue. By exporting lactate from intestine and erythrocytes, MCTl participates in the Cori cycle. It also participates in novel pathways of monocarboxylate metabolism in muscle and sperm.