The true thermodynamic activity (A(T)) Of cholesterol (Ch) in aqueous solutions containing taurocholate (TC)-Ch was determined by employing a direct assay of a 1 x 2-cm silicone polymer film with 0.025 cm thickness. Using the A(T) data, information on the nature of micellar species present in the TC-Ch system, and employing a binding-site model previously developed for tauroursodeoxycholate (TUDC)-Ch and taurochenodeoxycholate (TCDC)-Ch systems, it appeared that the Ch-binding affinity for simple bile-salt micelles corresponds precisely with the order of hydrophobicity, TUDC < TC < TCDC. Further, although simple TC micelles and simple TCDC micelles have similar binding capacities, the first Ch binding to a simple TC micelle may not significantly facilitate the second Ch binding, as occurs in simple TCDC micelles. For TUDC-Ch, TC-Ch, and TCDC-Ch systems, the concentration of bound simple micelles increased with increasing A(T) values, whereas the unbound simple micelle concentration decreased proportionally. These results provide insights into the possible influence of bile-salt species on Ch-binding to simple micelles in bile-salt-Ch solutions.