Quasi-elastic light-scattering methods were used to characterize micellar aggregates and microprecipitates formed in aqueous solutions containing sodium taurocholate (TC), egg lecithin (L), and cholesterol (Ch). Particle size and polydispersity were studied as functions of Ch mole fraction (XCh = 0-15%), L/TC molar ratio (0-1.6), temperature (5-85.degree. C), and total lipid concentration (3 and 10 g/dl in 0.15 M NaCl). For XCh values below the established solubilization limits (XChmax) added Ch has little influence on the size of simple TC micelles (type 1 systems), on the coexistence of simple and mixed TC-L micelles (type 2 systems), or on the growth of mixed disc TC-L micelles (type 3 systems). For supersaturated systems (XCh/XChmax > 1), 10 g/dl type 1 systems (L/TC = 0) exist as metastable micellar solutions even at XCh/XChmax = 5.3. Metastability is decreased in type 2 systems (0 < L/TC < 0.6), and labile microprecipitation occurs when XCh/XChmax exceeds .apprx. 1.6. In 10 g/dl mixtures, the microprecipitates initially range in size from 500-20,000 ANG and later coalesce to form a buoyant macroscopic precipitate phase. In 3 g/dl mixtures, the microprecipitates are smaller (200-400 .ANG.) and remain as a stable, noncoalesced microdispersion. Transmission electron microscopy of the microprecipitates formed at both concentrations indicates a globular noncrystalline structure, and lipid analysis reveals the presence of cholesterol and lecithin in a molar ratio (Ch/L) of .apprx. 2/1, suggesting that the microprecipitates represent a metastable cholesterol-rich liquid-crystalline phase. In supersaturated type 3 systems (0.6 < L/TC < 2.0), the precipitate phase is a lecithin-rich liquid-crystalline phase which likewise coalesces in a 10 g/dl system but forms stable vesicle (liposomal) structures (600-800 .ANG. radius) in 3 g/dl systems. In conjunction with these experimental data, a quantitative thermodynamic analysis of Ch solubilization in model bile systems is presented from which rigorous deductions of the free energy and enthalpy change for solubilization of cholesterol monohydrate in type 1 and type 2 systems are obtained. In addition, homogeneous nucleation theory is used to analyze the origin of the metastable/labile limit in supersaturated systems and to deduce the interfacial tension between microprecipitates and solution. On the basis of these experimental data and theoretical analyses, new hypotheses are offered on the structure and physiology of bile and the pathogenesis of Ch gallstones. In particular, it is suggested that the stable microprecipitates observed in 3 g/dl type 2 systems may provide a secondary vehicle (in addition to micelles) for cholesterol transport in supersaturated hepatic bile.