The synthesis of ampicillin catalyzed by Escherichia coli penicillin acylase was optimized in an aqueous system with partially dissolved antibiotic nucleus 6-aminopenicillanic acid (6-APA). The yields of both 6-APA and acyl donor could be improved by repetitively adding substrates to the reaction, allowing the concentration of 6-APA to remain saturated throughout. In this reaction concept, with four subsequent additions of substrates, 97% conversion of 6-APA and 72% of D-(-)-phenylglycine methyl ester (D-PGM) to ampicillin was achieved. The synthetic potential of this concept was estimated using a mathematical model which showed that by increasing the amount of added substrates a nearly quantitative conversion of 6-APA and 85% conversion of acyl donor into ampicillin could be achieved.