Previous methods such as equilibrium dialysis to measure chinoform binding to human serum albumin are flawed with problems due to the drug's poor solubility in neutral aqueous solutions and its strong adsorption onto the surfaces of dialysis membrane and other apparatus. Accordingly, we adopted a previously reported improved partition equilibrium method to keep off such difficulties, and developed additionally a rapid method using oil-wells. Both of these methods were used in this study. The equilibration of both partition and binding was attained after 6 h by the standard method and after 2 h by the rapid method. Two binding sites were found on the albumin molecule. One had an extremely large affinity constant of 10(8) M-1 and the other had a moderate one of 10(6) M-1. These values obtained with both methods agreed well and were sufficiently reproducible. The bound percentage of chinoform in serum was estimated at 99.999% under the condition where chinoform reveals its nervous toxicity. The binding constant had a maximum at pH 7.6, the physiological pH of serum. The pH profile suggested the participation of a histidine residue in the binding site. The inhibition experiment with palmitic acid revealed that the first binding site of chinoform was not identical with that of palmitic acid. The present method is applicable to analyses of protein binding by other drugs showing similar physicochemical properties as chinoform.