To provide an ABC(ATP-binding cassette) transporter inhibitor overcoming the multidrug resistance(MDR) to anticancer drugs due to the amplification and expression of MDR1 gene as the most important problem in cancer chemotherapy or the drug resistance to antimycotic drugs in mycosis.

The ABC transporter inhibitor comprises enniatins as active ingredient. This ABC transporter inhibitor, because of inhibiting drugs, especially anticancer drugs and antimycotic drugs, from being transported or excreted from cells into the outside of the cells, is useful for preventing or inhibiting the acquisition of resistance to various drugs.