Immunoassays, especially enzyme immunoassays (EIAs) and enzyme-linked immunosorbent assays (ELISA), are commonly used analytical techniques for clinical diagnostic measurements, drug screening, and measurements for evaluating exposure to environmental agents.(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16) ELISAs have been performed directly on diluted biological matrices(1, 2, 3) (blood, urine, saliva, breast milk, etc.), extracts from filters used for air sampling,(4) soil extracts,(5) food extracts,(6, 7) and myriad other varied matrices. The first ELISA, used to quantitatively measure immunoglobulin G (IgG), was described in 1971.(8) EIAs display accuracy comparable to that of traditional lab-based analytical methods, such as gas chromatography (GC). Their low cost, speed, and portability have made them attractive to environmental chemists in a variety of fields.(9) Immunoassays are based on the formation and detection of immune complexes between antigens (Ag) and antibodies (Ab). Antigens are principally macromolecules (e.g., proteins, polysaccharides, nucleic acids) that can act as complete immunogens able to stimulate an immune response. Other substances too small to act as immunogens on their own (drugs, pesticides, etc) must be coupled to a macromolecular carrier molecule (usually a protein) to become immunogenic and elicit an immune response. These small molecules are called haptens. Many environmental agents (such as pesticides or their metabolites) are haptens. In vivo, haptens may form adducts with constitutive proteins, becoming complete immunogens.