Several diagnostic tools are available for the identification of acute and latent viral infections. Although newly developed nucleic acid amplification methods, such as the polymerase chain reaction (PCR), have proved to be very useful diagnostic procedures, conventional methods, such as cell culture and serology, still play an important role in viral diagnostics. Despite the fact that modern serological assays, such as enzyme-linked immunosorbent assay (ELISA), are inexpensive and easy to perform, there is a strong demand to improve the performance of such systems. Most serological tests are based on poorly characterized antigens produced in infected culture cells. It has been shown, however, that only few viral antigens contained in these preparations are essential for serodiagnosis. In addition, numerous viral proteins display homologies with their counterparts from related viruses. Finally, the specificity of serological assays can also be reduced by contaminating proteins from host cells. Selective purification of natural viral antigens using, for example, immunoaffinity chromatography is one possible way to improve the quality of an antibody assay. However, the low concentration of most viral proteins in cell culture-derived antigen preparations reduces the practicability of this approach.