The polyene antibiotic filipin (a pentaene) has been studied using photophysical techniques. The polyene self-aggregates in water with a critical micellar concentration of 2 microM. Two approaches were used to evaluate the aggregate dimensions: (a) a lower limit of 10 nm for the aggregate radius was obtained from energy transfer experiments; (b) a formula for rationalizing the turbidity spectrum was derived, and from its application a spherical shape of radius about 50 nm was deduced. The low value for the fluorescence anisotropy of the aggregate (r = 0.02) is compatible with a very loose structure, i.e. the chromophore has very efficient depolarization dynamics that is not controlled by the aggregate size. The Stern-Volmer plot of aggregated filipin fluorescence quenching by iodide is non-linear, presenting a downward curvature. A model was used for the interpretation of these data, along with a study of the quenching in transient state; it was concluded that all the components of the decay are affected by the quencher, i.e. the aggregate has a very open structure with respect to the iodide ion. The partition constants of the polyene, Kp, between a model system of membranes (small unilamellar vesicles of dipalmitoylglycerophosphocholine) and the aqueous phase were determined from anisotropy measurements; the values obtained were Kp (gel phase) = (3.4 +/- 0.8) x 10(3) and Kp (liquid crystal phase) = (7.7 +/- 2.2) x 10(2). The observation that the polyene incorporation is efficient is at variance with the belief that the presence of sterols are essential for the interaction of polyene antibiotics with membranes [for review see Bolard, J. (1986) Biochim. Biophys. Acta 864, 257-304].