The binding of Mn( Ⅱ ) to human serum albumin (HSA) or bovine serum albumin (BSA) has been studied by equilibrium dialysis at physiological pH (7. 43). The Scatchard analysis indicates that there are 1.8 and 1.9 strong binding sites of Mn( Ⅱ ) in HSA and BSA, respectively. The successive stability constants which are reported for the first time are obtained by non-linear least-squares methods fitting Bjerrum formula. For both Mn( Ⅱ )-HSA and Mn( Ⅱ )-BSA systems, the order of magnitude of K1 was found to be 104. The analyses of Hill plots and free energy coupling show that the positive cooperative effect was found in both Mn( Ⅱ )-HSA and Mn( Ⅱ )-BSA systems . The results of Mn ( Ⅱ ) competing with Cu ( Ⅱ ) 、 Zn(Ⅱ)、Cd( Ⅱ) or Ca( Ⅱ ) to bind to HSA or BSA further support the conjecture that there are two strong binding sites of Mn( Ⅱ) in both HSA and BSA. One is most probably located at the tripeptide segment of N- terminal sequence of HSA and BSA molecules involving four groups composed of n The binding of Mn (II) to human serum albumin (HSA) or bovine serum albumin (BSA) has been studied by equilibrium dialysis at physiological pH (7. 43). The Scatchard analysis indicates that there are 1.8 and 1.9 strong binding sites of Mn (Ⅱ) in HSA and BSA, respectively. The developed stability constants which were reported for the first time were obtained by non-linear least-squares methods fitting Bjerrum formula. BSA systems, the order of magnitude of K1 was found to be 104. The analyzes of Hill plots and free energy coupling show that the positive cooperative effect was found in both Mn (II) -HSA and Mn (II) -BSA systems. The Results of Mn (Ⅱ) competing with Cu (Ⅱ), Zn (Ⅱ), Cd (Ⅱ) or Ca (Ⅱ) to bind to HSA or BSA further support the conjecture that there are two strong binding sites of Mn (Ⅱ) in both HSA and BSA. One is most probably located at the tripeptide segment of N-terminal sequence of HSA and BSA molecules inv olving four groups composed of n