The present research work addresses different aspects encompassing the synthesis of amoxicillin from p-OH-phenylglycine methyl ester and 6-aminopenicillanic acid (6-APA) as catalyzed by penicillin G acylase immobilized onto highly activated glyoxyl-agarose. A semi-empiric kinetic model is proposed and its parameters estimated. The main hypotheses upon which the model is based are: (i) antibiotic synthesis only occurs when 6-APA is previously bounded to the acyl-enzyme complex and 00 the rate of formation of this complex is not influenced by the presence of 6-APA. The model was validated for a wide range of initial substrate concentrations. It succeeded in describing the experimental data over a range typically used in industry, from 50 to 100 mM of 6-APA. To verify the postulated hypothesis, initial rates were determined at 25 degreesC, pH 6.5 and different substrate concentrations. The results obtained confirmed the first hypothesis (the need for a previous linkage of 6-APA to the enzyme) but the second one was not always valid. An activation or an inhibition effect was observed for some substrate concentrations. The semi-empiric model failed in the same conditions where these effects were observed. Nevertheless, in a wide range of substrate concentrations, including the ones of industrial interest, the model developed works perfectly well.