A method and apparatus for implementing the method is provided. The method involves performing an indirect competitive binding assay on a microarray to identify biological or chemical targets and screen for compounds of interest. The microarray comprises a common ligand located among membrane-, lipid- or protein-associated active binding sites. The method takes advantage of known or well-characterized binding kinetics, and steric interference between biological or chemicals targets of interest and universal readout units for different binding sites within the limited confines of a microspot. The biological targets, chemicals or organisms can specifically bind to target-binding sites, while the universal readout unit binds to the ligands in the microspot.