The disorganization and helix formation process of "Kunitz" soybean trypsin inhibitor (STI) effected by sodium dodecyl sulfate binding was investigated by the circular dichroism (CD) probe. The binding isotherms of dodecyl sulfate to STI were determined at the ionic strength of 0.033, 0.12, and 0.25 at pH 7.3, 25 degrees C. The perturbation and disorganization of this nonhelical protein were observed at an early binding stage (v, the average molar ratio of bound detergent to STI, up to about 7 in the case of the isotherm at I = 0.12). The disappearance of a positive CD peak at 226 nm and appearance of a negative CD band at 239 nm took place at this step and were affected by the number of carbon atoms in the alkyl group of detergents. The transition of the polypeptide backbone into a more ordered conformation proceeded gradually during cooperative binding of dodecyl sulfate molecules. An abrupt increase of detergent binding occurred near the critical micelle concentration of the detergent. The helix formation was completed prior to this step (v =30, at I = 0.12).