In this talk, I will describe ongoing efforts in my laboratory (in collaboration with Prof. Martin D. Burke at Illinois) to understand the mode of action of the gold standard antifungal drug amphotericin B (AmB). We have previously proposed a hypothesis that AmB acts as a sterol sponge, a high molecular weight assembly that cooperatively assembles and extracts ergosterol from the yeast plasma membrane. Binding of ergosterol is correlated with antifungal activity and binding of cholesterol with toxicity. Thus the sponge model predicts that analogs of AmB with greater binding specificity for ergosterol v. cholesterol will have an improved therapeutic index. The structural basis for this activity, however, remains incompletely understood. We have developed and implemented experiments involving 13C-labeled AmB both alone and in complex with sterols, in order to understand the detailed conformational rearrangements and structural motifs that endow AmB with these unique biophysical properties.