Chemical separations represent a large portion of the cost of bringing any new pharmaceutical product to the market. Membrane-based separation technologies(1,2), in which the target molecule is selectively extracted and transported across a membrane, are potentially more economical and easier to implement than competing separations methods; but membranes with higher transport selectivities are required, Here we describe an approach for preparing highly selective membranes which involves immobilizing apoenzymes within a microporous composite. The apoenzyme selectively recognizes its substrate molecule and transports it across the composite membrane, without effecting the unwanted chemical conversion of the substrate molecule to product, We demonstrate this approach using three different apoenzymes, Most importantly, it can be used to make enantioselective membranes for chiral separations, one of the most challenging and important problems in bioseparations technology, We are able to achieve a fivefold difference between the transport rates of D-and L-amino acids.