Various antigen-specific monoclonal antibodies (Mabs) are now being used clinically. Applications include in vitro testing (radioimmunoassay) and in vivo imaging (radioimmunoscintigraphy) for the early detection and staging of disease. Mabs can also be used as vehicles for delivering therapy (radioimmunotherapy) or to assess effects of therapeutic interventions. This review delineates those components making up antibody structure and discusses their functional significance. The method for radiolabeling immunoconjugates without altering their immunoreactivity or biological properties is dependent on an understanding of the structural units of the antibody and the labeling technique used. With the advent of genetic engineering, it has become feasible to design antibodies to circumvent certain adverse features or enhance a certain property. A review of the various "designer" antibodies and their relative advantages in the clinical setting is presented.