This chapter reviews the structure of viral antigenic determinants and describes the antigenic properties of two genera of plant viruses, the tobamoviruses and the potyviruses. In order to apply serological techniques in plant virology in the most efficient manner, it is useful to have some insight into our current knowledge of protein antigenicity. The serological properties of potyviruses have been found extremely useful for identifying and classifying the individual members of this very large group of plant viruses. Virus preparations used for the production of polyclonal antiserum should be free of contaminating plant antigens. When enzyme-linked immunosorbent assay (ELISA) is used to measure distant serological relationships, the concentration of the reactants should be adjusted to allow the most sensitive detection of antibody. In many virus-host combinations, crude plant extracts contain enough virus to allow ELISA plates to be coated directly with sap from infected plants.