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From measurements of the autocorrelation function and time-averaged intensity of light scattered from aqueous bile salt-lecithin solutions, we deduced the mean hydrodynamic radius (Rh), shape, and polydispersity of bile salt-lecithin mixed micelles as functions of bile salt species, lecithin to bile salt (L/BS) molar ratio, total lipid concentration (0.625-10 g/dL), temperature (20-60 degrees C), and NaCl concentration (0.15-0.6 M). Our data suggest that at low L/BS ratios (0 to approximately 0.6) simple bile salt micelles coexist in varying proportions with minimum-sized mixed micelles (Rh, 18-35 A). These solutions are highly polydisperse and display features dependent upon the particular bile salt species. At high L/BS ratios (greater than 0.6), only mixed micelles are present, and their sizes increase markedly (Rh, 20 leads to 300 A) with increases in L/BS ratio and appear to diverge as the lecithin-bile salt phase limit is approached. The shape of the mixed micelles as deduced from light-scattering measurements and confirmed by transmission electron microscopy is disklike. The radii of the disks, however, are not compatible with Small's model of mixed micellar structure [Small, D.M. (1967a) Gastroenterology 52, 607-a1 but are consistent with a new model proposed here in which bile salts and lecithin interact to form a mixed bilayer disk which is surrounded on its perimeter by bile salts. The inclusion of bile salts in a fixed stoichiometry within the interior of the bilayers is shown to provide a quantitative explanation for the divergence of the mixed micellar sizes, their temperature dependence, and the origin of the lecithin-bile salt phase limit. The influence of total lipid concentration on both mixed micellar size and the lecithin-bile salt phase limit is explained by the "mixed disk" model by taking account of the equilibrium between mixed micelles and bile salt monomers in the intermicellar solution. By use of this concept, deductions of the intermicellar bile salt concentration in taurocholate-lecithin solutions are made and are shown to vary as a function of mixed micellar size and temperature. The range of values obtained, 3-6 mM, is comparable in magnitude to the critical micellar concentration of the pure bile salt.