Mammalian cells accumulate reduced folates and methotrexate, a folate antagonist, through the reduced-folate carrier (RFC) (Goldman, I.D., Lichtenstein, N.S., and Oliverio, V.T. (1968) J. Biol. Chem. 243, 5007-5017). This study describes the isolation and expression of a cDNA clone that restores RFC activity to human breast cancer cells defective in this transporter. The cDNA cancer cells defective in this transporter. The cDNA codes for a peptide (mRFC1) of 58 kDa, whose hydropathy plot, resembling those of mammalian sugar transporters, predicts that it may be a member of a superfamily of transporter genes. Transfection of methotrexate-resistant (MTXR) ZR-75-1 cells with an expression vector, pRFC1, that codes for this peptide restores their ability to accumulate methotrexate. Furthermore, transport of methotrexate into pRFC1-transfected cells is blocked by a 10-fold molar excess of the reduced folate, 5-formyltetrahydrofolic acid, but is unaffected by folic acid. The increase in methotrexate uptake that is observed in pRFC1-transfected MTXR ZR-75-1 cells reverses their resistance to this antitumor agent.