Bilirubin is a major component of gallstones and the solubility of unconjugated bilirubin (UCB) has been shown to play an important role in gallstone formation. The mechanism of gallstone formation and solubilization of UCB is, however, still unknown. Several investigators have attempted to determine the chemical nature of bilirubin which are thought to be closely related to its solubility. Hoivever, there is some controversy over the ionization constants and pK values of carboxyl groups of UCR. In the present study, the effect of pH, the type and concentration of bile salt on UCB solubility were studied and the pK values of bilirubin were calculated. The results were as follows: 1) There are more solubilities with higher bile salt concentrations and with increasing pH, and bile salt plays an important role in solubilizing UCB in bile. 2) Bile salt monomers, dimmers and micelles are equally effective in solubilizing bilirubin. 3) The bilirubin in bile is almost entirely in the monoanion (BH- ) form at physiological pH values.