A notable hysteretic effect has been observed in the interaction of Ni^2^+ ion with human or bovine serum albumin using UV-Visible spectrometry, which shows that the binding of Ni^2^+ ion can induce a slow transition of HSA and BSA from the conformation of weaker affinity for Ni^2^+ ion to the one of stronger affinity (T-R transition). This conformational transition is supported by the time-dependence of the optical rotation of the samples. The rate constants and activation parameters of these transitions have been measured and discussed. It is inferred that such a conformational transition may mainly occur in the IA subdomain of the proteins, and is likely to be a "hinged movement", which makes the IA subdomain become more open.

Key words: ULTRAVIOLET SPECTROPHOTOMETRY, NICKEL, SERUM PROTEINS, HYSTERETIC EFFECTS