Gel-permeation, high-performance liquid chromatography (GP-HPLC) studies were conducted to quantify simple micelles and mixed micelles present in model biles. A TSK-gel G2000SW column was preequilibrated with a buffered mobile phase : phosphate buffer, pH 7.4, 0.1 M NaCl, and 30 mM [C-14]taurocholate (TC). Simulated biles with different TC and/or egg-yolk lecithin (L) levels were then introduced onto the GP column. The injected biles without L (TC alone) yielded positive peaks or negative valleys (relative to the baseline) at similar to 9.5 min, depending on the injected TC concentration. A theoretical mass balance equation quantitatively accounted for the observed simple micelle peaks (or valleys). When TC-L coexisting simple micelle-mixed micelle model biles were injected onto the column, in addition to the simple micelle peaks at similar to 9.5 min, mixed micelle peaks were observed at retention times in the region similar to 7.5 to 8.0 min. Here, also, there was good mass balance between the expected and the observed TC peaks. The average molecular weights were estimated as similar to 28,000-45,000 Da for the mixed micelles and similar to 5500 Da for the simple micelles, respectively. The TC-to-L molar ratio (K) for the mixed micelle peak showed a curvilinear decrease as a function of L concentration and leveled off at around 1.5. These results are consistent with those obtained from previous equilibrium dialysis studies, although the K values from present studies are slightly lower (similar to 20%); a possible explanation for this discrepancy is presented.