Serological analysis of tumor antigens by recombinant cDNA expression cloning (SEREX) allows the systematic cloning of tumor antigens recognized by the spontaneous autoantibody repertoire of cancer patients. For SEREX, cDNA expression libraries are constructed from fresh tumor specimens, packaged into λ-phage vectors, and expressed recombinantly in Es-cherichia coli. Recombinant proteins expressed during the lytic infection of bacteria are transferred onto nitrocellulose membranes to be probed with diluted autologous patient serum for identification of clones reactive with high-titered IgG antibodies. This chapter describes the SEREX technology in detail.