In an effort to develop new functionalities of naturally occurring ionophores, we designed a new "biomimetic membrane transport" system mediated by monensin, lasalocid, and salinomycin. Under nonbiological conditions, these ionophores specifically transported unnatural guest species such as amino acid ester salts and heavy and transition metal cations. Liquid-liquid extraction and C-13-NMR binding experiments clearly demonstrated that these biological ionophores accommodated various guest cations in their pseudocavities and that the overall transport rate was significantly determined by a combination of rates of guest binding and releasing processes. Several biomimetic applications can be envisaged using these ionophores in the border regions between synthetic and biological supramolecular chemistry.