The interaction of amphotericin B with ergosterol was studied in aqueous solutions of propanol. The mode of the interaction was found to be related to the aggregation state of amphotericin B. Ergosterol does not react (or reacts extremely slowly) with monomeric amphotericin B. Traces of a small aggregate, probably a dimer, enable a cooperative reaction. At high concentrations of the dimer, the reaction is immediate and the concentration of amphotericin B complexed with ergosterol is twice as high as the amount of added sterol. The interaction with ergosterol is hindered when the antibiotic is in micellar form. The pharmaceutical form, Fungizone, behaves similarly to the pure amphotericin B. Fungizone's greater solubility in water does not modify either the extent or the mode of interaction with ergosterol.