The mechanism of complex formation between protamine and a double-stranded DNA (dsDNA) is studied by means of microcalorimetry and spectsopolarimetry. From the calorimetric and spectral results, a possible driving force leading to complex formation based on the interaction of the dsDNA with salmine is dominated by cooperative actions between two factors : the first is the electrostatic interaction between the PO4- group in the main chain of the dsDNA and the C(NH2)(2)(+) group of the arginine residue in salmine, and the second is the effect of dehydration accompanying the dsDNA-salmine complex formation induced by the reverse transition from a coiled conformation to a helical one of salmine. To obtain information concerning the conformational change of salmine accompanying the dehydration by interacting with the dsDNA, DSC measurements of salmine solutions with various concentrations of 2,2,2-trifluoroethanol added to salmine with a coiled conformation in aqueous solutions are carried out. The change in enthalpy, Delta H-c-b, for the reverse transition from a coiled conformation to a helical one of salmine is estimated to be about 2.1 kJ/(mol arginine residue). From the enthalpy cycle, the enthalpy change, Delta H-dehy, of the dehydration accompanying the interaction is also estimated to be about 6.7 kJ. We suggest that when the salmine molecule having a random coiled conformation binds to the dsDNA, the salmine molecule transforms into the alpha-helical form and is able to bind along the groove of the dsDNA and its enthalpy change is estimated to be about -11.0 kJ/(mol arginine) as the electrostatic interaction involves the C(NH2)(2)(+) group of the arginine residue with a positive charge in salmine and the PO4- group with a negative charge in the main chain of the dsDNA, taking into consideration Delta H-c-h and Delta H-dehy values.