CT antigen–specific antibody responses in patients with multiple myeloma following allogeneic stem cell transplantation. Sera from patients with MM (n = 66) were screened for IgG antibodies against 3 CT antigens (MAGEA3, SSX2, NY-ESO-1) in an ELISA. Sera showing positive results were sent to the New York branch of the Ludwig Institute for Cancer Research for confirmation of the results using recombinant proteins of 10 different CT antigens (NY-ESO-1, LAGE1, MAGEA1, MAGEA3, MAGEA4, MAGEA10, SSX1, SSX2, SSX4, XAGE1) and p53 in an ELISA. Optical density (OD) results (serum dilution 1:100) of the ELISA are given for 3 representative antibody-positive patients with MM, with patients BMT65 and BMT17 showing high-titered antibodies against SSX2, and patient BMT47 showing a strong antibody response against NY-ESO-1 and its homologue LAGE following allogeneic stem cell transplantation. Importantly, pre–bone marrow transplantation samples of all patients who had undergone alloSCT were negative for CT antigen–specific IgG antibodies.