zeta -potential and UV difference spectroscopy techniques have been utilized to study the interaction of the amphiphilic drug propranolol hydrochloride with human haemoglobin (KH) and human serum albumin (HSA) in aqueous solution at pHs below, above, and at the isoelectric points of both proteins at 25 degreesC. The number of adsorption sites on both proteins was determined from the observed increases of the zeta -potential as a function of drug concentration in the regions of positive zeta -potential, where the adsorption was a consequence of the hydrophobic effect. The Gibbs energies of adsorption of the drug onto the proteins showed an exponential decrease vith increase of drug concentration. The interactions between HSA-propranolol complexes in the presence of propranolol hydrochloride were interpreted from dynamic lightscattering data using the Derjaguin-Landau-Venvey-Overbeek (DLVO) theory. The concentrations of the amphiphilic drug were maintained below its critical concentration, so its behavior could be considered to be that of a 1:1 electrolyte. This treatment has been widely used to obtain interactions between micelles in electrolyte solution but to our knowledge has not previously been applied to systems of the type considered here.