Purine arabinosides are well known antiviral and antineoplastic drugs. Since their chemical synthesis is complex, time-consuming, and polluting, enzymatic synthesis provides an advantageous alternative. In this work, we describe the microbial whole cell synthesis of purine arabinosides through nucleoside phosphorylase-catalyzed transglycosylation starting from their pyrimidine precursors. By screening of our microbial collection, Citrobacter koseri (CECT 856) was selected as the best biocatalyst for the proposed biotransformation. In order to enlarge the scale of the transformations to 150 mL for future industrial applications, the biocatalyst immobilization by entrapment techniques and its behavior in different reactor configurations, considering both batch and continuous processes, were analyzed. C koseri immobilized in agarose could be used up to 68 times and the storage stability was at least 9 months. By this approach, fludarabine (58% yield in 14h), vidarabine (71% yield in 26h) and 2,6-diaminopurine arabinoside (77% yield in 24h), were prepared. (C) 2012 Elsevier Ltd. All rights reserved.