Valinomycin is a natural, lipid-soluble molecule (Mr = ~1,110) that binds potassium ions (K+) and facilitates their transfer across lipid bilayers (i.e., it is an ionophore). It is a macrocyclic molecule formed from 12 alternating amino acids and esters, synthesized by specific Streptomyces. Valinomycin exhibits a 10,000-fold selectivity for K+ over Na+. Valinomycin is a potent antibiotic. It is commonly used as an agent to induce apoptosis and can also be used to calibrate potentiometric responses in membrane experiments involving potential-sensitive dyes such as di-4-ANEPPS.