The interaction of a bile salt, sodium deoxycholate, with insulin was investigated by ultraviolet derivative spectroscopy, fluorescence, circular dichroism and photon correlation spectroscopy. The results indicate that the conversion of insulin from a monomer to a dimer, tetramer, or hexamer occurs over a concentration range of 5.5x10(-2)-1.1x10(-1) mg/ml in alkaline solution in the absence of bile salts, and that insulin exists primarily as a hexamer complex above this concentration. Evidence is presented that bile salt-induced dissociation of insulin at a high concentration is driven by electrostatic repulsion and hydrophobic interaction resulting from bile salt binding, and occurs at a concentration lower than the critical micelle concentration of bile salts. We suggest that the interaction between insulin and the bile salt occurs by hydrophobic binding followed by a poorly cooperative binding process with an increasing concentration of bile salts. In addition, the bile salt binding to the surface of insulin alters not only the microenvironment around tyrosine residues but also the conformation of insulin to some degree, which probably resulted from the dissociation of insulin. However, no notable loss of structure happens to insulin during all the processes under our experimental conditions. A mechanism of bile salt binding to insulin and bile salt-mediated dissociation of insulin oligomers is proposed whereby hydrophobic interaction and electrostatic repulsion happen between them. This work also showed that the combination of these spectroscopic studies is a powerful tool to clarify the interactions between these surfactants and proteins.