The interactions between human serum albumin (HSA) and two anionic amphiphilic penicillins, sodium cloxacillin and dicloxacillin, in aqueous solution of pH 4.5 (ionic strength 0.05 M) and 7.4 (ionic strength 0.17 M) at 298.15 K have been determined by surface tension measurements. The surface behavior for both penicillins in such buffered solutions was also analyzed for comparison. Critical micelle concentrations (cmc's) for both drugs were obtained. Measurements in the presence of a mass of HSA per unit volume of 0.125% were carry out under conditions where the protein molecule was positively or negatively charged. In the absence of HSA and for both pHs, lower values of the cmc for dicloxacillin reflect its greater hydrophobicity. The presence of the protein in solution resulted in a decrease of the surface tension and in an increase of the cmc's of the penicillins, suggesting the formation of surface-active complexes. Their adsorptions at the air/solution interface have been calculated and compared.