Surface plasmon resonance (SPR) biosensors recently gained an important place in drug discovery. Here we present a primary and secondary SPR biosensor screening methodology. The primary screening method is based on a direct binding assay with covalent immobilized drug target proteins. For the secondary screening method, a sequential competition assay has been developed where the captured protein is first exposed to an unknown test compound, followed directly by an exposure to a high-molecular-weight reporter ligand. Using the high-molecular-weight reporter ligand to probe the remaining free binding site on the sensor, a significant signal enhancement is obtained. Furthermore, this assay format allows the validation of the primary direct binding assay format, efficiently revealing false positive data. As a model system, acetylcholine binding protein (AChBP), which is a soluble model protein for neuronal nicotinic acetylcholine receptors, has been used. The secondary assay is lower in throughput than the primary assay; however, the signal-to-noise ratio is two times higher compared with the direct assay, and it has a z' factor of 0.96. Using both assays, we identified the compound tacrine as a ligand for AChBP. © 2010 Elsevier Inc.