This chapter focuses on the chemical aspects of enzyme-immunoassay: the preparation and characterization of enzyme-hapten conjugates and enzyme-protein conjugates. It reviews protein-protein coupling, chemical cross-linking of proteins, enzyme-immunoassay, and drug immunoassays. The chemical methods currently employed in the development of enzyme-immu-noassays are largely borrowed from the fields of peptide chemistry and chemical modification of proteins. The reagents which have been used for protein modification in enzyme-immunoassay work may be conveniently classified into the following broad categories: Acylating reagents, Alkylating reagents, Oxidizing and reducing reagents and Electrophilic reagents. The methods for coupling haptens to carrier proteins for immunization, and to enzymes for use as immunoassay reagents, are generally similar. They vary according to the structure of the hapten and nature of its functional groups. Many methods are available for introducing carboxyl groups into haptens. For example, alkylation of oxygen or nitrogen substituents with halo-esters, followed by ester hydrolysis, has been frequently used.