Resistance to chemotherapy limits the effectiveness of anti-cancer drug treatment. Here, we present a new approach to overcome the setback of drug resistance by designing a conjugate of a cell-penetrating peptide and the cytostatic agent methotrexate (MTX). Two different peptides, YTA2 and YTA4, were designed and their intracellular delivery efficiency was characterized by fluorescence microscopy and quantified by fluorometry. MTX was conjugated to the transport peptides and the ability of the peptide-MTX conjugates to inhibit dihydrofolate reductase, the target enzyme of MTX, was found to be 15 and 20 times less potent than MTX. In addition, in vitro studies were performed in a drug resistant cell model using the 100-fold MTX resistant breast cancer cells MDA-MB-231. At a concentration of 1 microM, the peptide-MTX conjugates were shown to overcome MTX resistance and kill the cells more efficiently than MTX alone. Estimated EC50's were determined for MTX, MTX-YTA2 and YTA2 to be 18.5, 3.8 and 20 microM, respectively. In summary, cell-penetrating peptide conjugation of MTX is a new way of increasing delivery, and thereby, the potency of already well-characterized therapeutic molecules into drug resistant tumour cells.