The ionophore properties of amphotericin B (AmB) are related to the transport of Na+ and K+ ions across the molecular pores formed by this antibiotic in lipid membranes. In this paper, we present a new, complementary mechanism in which the -COO- group of the antibiotic is involved in the binding process of Na+ and K+ ions. Spectroscopic studies indicate that K+ and Na+ ions play an important role in the AmB aggregation process. Evidence in several spectral regions shows that K+ ions exhibit a stronger ionic binding affinity to the -COO- group relative to Na+. Overall, our findings indicate that monovalent ions can affect the molecular organization of AmB in substantially different ways not previously considered to be significant for their biological action.