Immunoprecipitation consists of multiple ordered steps: lysing the cell with detergent if the antigen (usually a protein) to be precipitated is membrane-bound; binding of a specific antigen to an antibody; precipitating the antibody-antigen complex; washing the precipitate; and dissociating the antigen from the immune complex. The dissociated antigen is then analyzed by electrophoretic methods. In this unit, the basic protocol details the immunoprecipitation of a radiolabeled antigen with a specific antibody (polyclonal or monoclonal) covalently linked to Sepharose. Preparation of Ab-Sepharose is described in the Support Protocol. The first two alternate protocols present methods for precipitating or isolating the soluble immune complexes formed between a specific antibody and a radiolabeled antigen. Immunoprecipitation is achieved with polyclonal anti-immunoglobulin (Ig) serum, anti-Ig-Sepharose, Staphylococcus protein A or Streptococcus protein G bound to Sepharose, or Staphylococcus aureus bacteria which contain protein A on the cell surface. The third alternate protocol should be used for immunoprecipitation of antigens that are nonspecifically associated with other proteins. The fourth alternate protocol describes immunoprecipitation of unlabeled protein antigens with Ab-Sepharose.