Mixed Langmuir monolayers of amphotericin B (AmB) and dipalmitoyl phosphatidyl choline (DPPC) were investigated by recording surface pressure-area (pi-A) isotherms in addition to Brewster angle microscopy. The analyses of pi-A and compressional modulus curves indicate the existence of interactions in the AmB-DPPC system; the greatest were found to occur for a ca. 2:1 AmB/DPPC mixture. For this mixed monolayer (X-AmB = 0.66), the formation of a stable complex, composed of two horizontally oriented AmB molecules and one DPPC molecule in a vertical position, is suggested. For mixtures containing AmB in excess as compared to the stoichiometric mixture for the complex formation (X-AmB > 0.66), the film-forming components are miscible and the mixed monolayer consists of AmB-DPPC complexes together with horizontally (at surface pressures below the first transition) or vertically (at pressures above this transition) oriented free AmB molecules. On the other hand, mixtures of AmB-DPPC complex and DPPC in excess (at X-AmB < 0.66) were found to be miscible at surface pressures below the second transition. However, the system becomes immiscible at higher surface pressures.