The protein binding of warfarin in serum has been studied by means of circular dichroism and equilibrium dialysis. Evidence was found that the N-B transition of albumin, occurring around physiological pH, takes place not only in solutions of pure albumin but also in serum. The protein binding of warfarin in serum is pH-dependent and increases with pH especially around physiological pH. This pH-dependent serum binding of warfarin can be reasonably explained by the N-B transition of albumin. The effect of Ca2+ and Mg2+ on the protein binding of warfarin in serum is negligible at pH 7.4, whereas at this pH Cl- increases the free-warfarin concentration by a competitive displacement.