Methods and devices for the parallel, in vitro screening of biomolecular activity using miniaturized microfabricated devices are provided. The biomolecules that can be immobilized on the surface of the devices of the present invention include proteins, polypeptides, nucleic acids, polysaccharides, phospholipids, and related unnatural polymers of biological relevance. These devices are useful in high-throughput drug screening and clinical diagnostics and are preferably used for the parallel screening of families of related proteins.