On the basis obtained in a preceding study, cholesterol solubilization in aqueous bile salt-lecithin solution was investigated. The alteration of mixing sequence was found to yield differences not only in the rate but also in the magnitude of cholesterol solubilization. Both the rate and the magnitude were remarkably bigger in the system solubilizing cholesterol and lecithin mixture by bile salt than that solubilizing cholesterol crystal by bile salt with solubilized lecithin. A linear relation between the quantity of solubilized cholesterol and the concentration of bile salt except for lower concentration range was obtained for every bile salt-lecithin system. Values of k, the slope of the partial straight line, determined for cholate, chenodeoxycholate, deoxycholate and equimolar cholate-deoxycholate systems were 5.85 X 10(-2), 7.60 X 10(-2), 9.50 X 10(-2) and 7.17 X 10(-2), respectively. Cholesterol solubilizing power of bile salt was thus enhanced by the addition of lecithin. Since the solubilizing power could be given by the ratio of the solubilizate to the solubilizer, it was expressed graphically by the ratio of cholesterol to bile salt as ordinate and the concentration of bile salt as abscissa. The saturability of cholesterol solubilization in bile was purposefully exhibited in this graph by plotting assayed data of biliary lipid components.