Analysis of the compression isotherms of ergosterol/amphotericin B (AmB) mixed monolayers spread on aqueous substrates shows the existence of interactions between the two components at AmB mole fractions between 0.1 and 0.7. At low surface pressure AmB molecules appear to lie horizontally in the A/W interface and ergosterol molecules to stand vertically, while at higher surface pressures the molecules of both components are vertically oriented at the interface.