Biosensor instruments, such as the BIACORE, are gaining popularity for analysing serum samples for the presence of antibodies. These instruments offer several advantages in the detection and subsequent characterization of clinically relevant antibodies generated in response to administration of therapeutic proteins. Much like other common immunoassay platforms, immobilized ligand is used to capture antibodies. Unlike conventional approaches, the ligand is immobilized to the surface of a biosensor chip, with detection based upon surface plasmon resonance. This assay platform, therefore, does not require reporter molecules such as enzymes, fluorochromes or radioisotopes that are common to conventional immunoassay methodologies. Additional desirable features of the biosensor platform include real-time detection of the binding of antibody to ligand (for kinetic measurements) as well as straightforward characterization of antibody isotype, specificity and relative concentration. This is all performed with minimum serum requirements (typically 10 microlitres per sample analysed) in a fully automated environment. The unique features of the biosensor instrument warrant that these assays are referred to as biosensor immunoassays to clearly distinguish them from more conventional immunoassay methodologies, such as ELISA.