The interaction of myelin basic protein (MBP) from bovine central nervous system with divalent copper ion was studied by equilibrium dialysis and isothermal titration calorimetry techniques at 27 degreesC in Tris buffer solution at pH = 7.2. MBP has two binding sites for copper ion. The intrinsic association equilibrium constants are 0.083 and 1.740 muM(-1) in the first and second binding sites, respectively. Hence, occupation of the first site has produced an appreciable enhancement 21 of the binding affinity of the second site. A new representation of titration calorimetric data, as well as, the Scatchard plot, shows positive cooperativity in two binding sites for copper ions. The molar enthalpies of binding are -13.5 and -14.8 U mol(-1) in the first and second binding sites, respectively.