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Cross-linked enzyme crystals (CLECs®) are a novel form of immobilised biocatalyst designed for application in large-scale biotransformation processes. In this work we review the production and characterisation of CLECs® prepared from three enzymes (yeast alcohol dehydrogenase I (YADHI), Candida rugosa lipase and α-chymotrypsin) over a range of crystallisation and cross-linking conditions. Optimisation and control of the crystallisation process, with respect to crystal form and enzyme activity yield, was facilitated by the use of triangular crystallisation diagrams which allowed three parameters (e.g. protein concentration, precipitant concentration and pH) to be varied simultaneously. These diagrams showed regions, or 'crystallisation windows', in which particular crystal forms or optimal activity recoveries (up to 87%) could be obtained. They also identified conditions for reproducible scale-up of the lipase crystallisation from 0.5 to 500 mL scale.In order to evaluate the suitability of a particular bat...