The complexes formed by the interaction of the amphiphilic drug amitriptyline hydrochloride and human serum albumin (HSA) in aqueous solution at pH 3.2, 4.9 (the isoelectric point), and 6.0, were investigated at 25 degreesC using a range of physicochemical techniques. The complexation process was investigated by conductometric measurements on HSA/amitriptyline solutions. of increasing drug concentration from which values of the critical concentration at which adsorption of drug commenced and also the critical micelle concentration of amitriptyline in the presence of protein were determined. The number of adsorption sites was determined from the observed increases of the xi-potential as a function of drug concentration in the regions of positive xi-potential where the adsorption was a consequence of the hydrophobic effect. Gibbs energies of adsorption of the drug onto the protein showed an exponential decrease with increase of drug concentration. Measurements of the molar mass and hydrodynamic radius of the HSA-amitriptyline complexes as a function of drug concentration by static and dynamic light-scattering techniques have shown a gradual increase of size typical of a saturation rather than denaturation process. Plots of hydrodynamic radius against amitriptyline concentration have been interpreted by comparison with typical binding isotherms.