alpha-Amino acid ester hydrolases (AEHs) are enzymes of interest to the semi-synthesis of beta-lactam antibiotics with alpha-amino, such as cephalexin and cefaclor. An undesired side reaction, the hydrolysis of alpha-amino acid ester, had hindered applications in antibiotics synthesis. Although the enzymes' S/H ratio can be increased by protein engineering, such approaches require a suitable screening assay. Such a screening assay has not yet been described for AEHs. In this paper, we report a 96-well plate format screening procedure for AEHs based on two spectrophotometric assays. To reduce the hydrolysis reaction while maintaining synthesis activity, and to evaluate the effectiveness of the screening strategy, we introduced random mutations in part of the aeh gene from Xanthomonas rubrillineans by error-prone PCR. By a parallel plate-based screening strategy, three mutants with improved S/H ratio, R87L, T132N and N219I, were obtained. (c) 2012 Elsevier Inc. All rights reserved.