Analysis of the humoral immune response to infectious diseases has played, and will to continue to play, a key role in their diagnosis and immune surveillance. Although rapid genome detection methodologies, such as PCR, are beginning to replace immune assays for disease diagnosis, they are not suitable for all applications, especially the surveillance of the immune status of human populations. Here we review the limitations of current conventional tools for measuring immune responses and outline principles for the design and production of novel diagnostic reagents. Methods for the production of viral diagnostic antigens by a variety of recombinant systems are described and their relative merits and disadvantages discussed. Protocols for the production of viral diagnostic antigens in eukaryotic, insect and mammalian systems are described using measles nucleocapsid antigen as a model. Indirect ELISA protocols which can differentiate immunoglobulin classes and subclasses are also described. Examples of the use of these analyses in research and surveillance are given.