One of the many challenges in drug discovery process is to identify molecules from millions of compounds that bind tightly to biological target. This step usually involves screening libraries with targetspecific functional (cell-based or in vitro biochemical) assays and synthesizing structural analogs to optimize these leads. Many biochemical assays including scintillation proximity assay, fluorescence polarization, colorimetric assay, membrane permeation assay, affinity capillary electrophoresis, solid phase assay, and enzyme-linked immunosorbent assay (ELISA) have been successfully developed . In this report, we describe the principle of ELISA and its applications to combinatorial chemistry.