zeta-Potential measurements have been used in studying the interaction of a range of penicillins, nafcillin, cloxacillin, dicloxacillin, and flucloxacillin with human serum albumin (HSA) in aqueous solution at pH 7.4 at 25 degrees C. The zeta-potentials of HSA became more negative as the negatively charged penicillins bind to it. The variations of zeta-potential with drug concentration have been interpreted in terms of the theory of Ottewill and Watanabe (OW) (Kolloid-Z. 1980, 170, 132). The theory has been modified to take into account the cooperativity of drug binding in terms of the Hill equation. The Hill coefficients for the drugs are all greater than unity and diagnostic of positive cooperativity on binding. By application of both the OW theory and its modified form, Gibbs energies per drug molecule bound (Delta G(<(nu)over bar>)) as a function of the number of drug molecules bound per protein molecule were obtained. These plots are of the form and magnitude similar to those obtained from direct binding measurement, using the equilibrium dialysis technique. The application of the Hill equation to adsorption is consistent with clustering of drug molecules in micelle-like aggregates to the polypeptide chain of the protein.