Abstract: Methods for the parallel, in vitro screening of biomolecular activity using miniaturized microfabricated devices are provided. The biomolecules immobilized on the surface of the devices employed in the method of the present invention include proteins, polypeptides, polynucleotides, polysaccharides, phospholipids, and related unnatural polymers of biological relevance. These methods are useful in drug development, functional proteomics studies and clinical diagnostics and are preferably used for the parallel screening of families of related proteins.