Immunoprecipitation allows the investigator to detect and quantitate antigens in a mixture of proteins or characterize a specific antibody response to already well-characterized proteins. Addition of antibodies to proteins, usually radiolabeled, allows formation of antigen—antibody complexes. After separation from contaminating proteins, the complexes are disassociated and the proteins of interest are separated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). Size and quantity of proteins may then be analyzed either by autoradiography or a gel scanning procedure. Immunoprecipitation is extremely sensitive and may detect very small amounts of radiolabeled protein (detection level ~100 pg protein or 100 cpm/protein). Unlabeled proteins may be used if other sensitive detection methods are utilized, e.g., enzymatic activity assays or Western blotting. The advantage of the immunoprecipitation technique vs immunoblotting is the possibility to analyze the immune response to proteins expressed in their native conformation. Radioimmunoprecipitation assay (RIPA) is used routinely for the detection of viral proteins, characterization of monoclonal and polyclonal antibody preparations, and determination of the specificity of the immune response to various pathogens (1–3).