This work compared the performance of three hydrophobic resins for the adsorption of ampicillin (AMP), D-phenylglycine (PG), D-phenylglycine methyl ester (PGME), and 6-aminopenicillanic acid (6-APA). The influence of pH on adsorption efficiencies was assessed in the range of 4.5-8.5, at 4 and 25degreesC. The values at 4degreesC were slightly higher than those at 25degreesC. The adsorption efficiency of AMP and 6-APA decreased at higher pHs, for the three resins. An opposite behavior was found for PGME, and the pH did not affect PG adsorption efficiency. Isotherm models were fitted to experimental equilibrium data and the best models were discriminated.