The generation of high-affinity antibodies (Abs) against hapten targets (molecular weight below 1000 Dalton) presents particular problems not encountered with larger antigens (Ags). By their nature, haptens are invisible to the host immune system unless presented as an epitope conjugated to a suitable immunogenic carrier protein, such as bovine thyroglobulin. The principal interest in anti-hapten Abs is as detection molecules for use in diagnostic assays. These typically use dipstick (qualitative) or, more commonly, enzymelinked immunosorbant assay (ELISA) formats, for the quantification and/or detection of targets such as environmental pollutants or for monitoring the presence of drugs in clinical samples. There are also applications related to biological functions, e.g., Abs directed against signal molecules enhance the study of cell signaling pathways and have potential as candidate therapeutic agents.