Langmuir isotherm, neutron reflectivity, and Brewster angle microscopy experiments have been performed to study the interaction of amphotericin B (AmB) with monolayers prepared from 1-palmitoyl,2-oleoyl-phosphatidylcholine (POPC), and mixtures of this lipid with cholesterol or ergosterol to mimic mammalian and fungal cell membranes, respectively. isotherm data show that AmB causes a more pronounced change in surface pressure in the POPC/ergosterol system than in the POPC and POPC/cholesterol systems and its interaction with the POPC/ergosterol monolayer is also more rapid than with the POPC and POPC/cholesterol monolayers. Brewster angle microscopy shows that, in interaction with POPC monolayers, AmB causes the formation of small domains which shrink and disappear within a few minutes. The drug also causes domain formation in the POPC/cholesterol and POPC/ergosterol monolayers; in the former case, these are formed more slowly than is seen with the POPC monolayers and are ultimately much smaller; in the latter case, they are formed rather more quickly and are more heterogeneous in size. Neutron reflectivity data show that the changes in monolayer structure following interaction with AmB are the same for all three systems studied: the data are consistent with the drug inserting into the monolayers with its macrocyclic ring intercalated among the lipid acyl chains and sterol ring systems, with its mycosamine moiety co-localising with the sterol hydroxyl and POPC head groups. On the basis of these studies it is concluded that AmB inserts in a similar manner into POPC, POPC/cholesterol and POPC/ergosterol monolayers, but does so with differing kinetics, and with the formation of quite different in-plane structures. The more rapid time-scale for interaction of the drug with the POPC/ergosterol monolayer, its more pronounced effect on monolayer surface pressure, and its more marked changes as regards domain formation, are all consistent with the drug's selectivity for fungal vs. mammalian cell membranes.