Mice immunized with embryonic stem cells produce antibodies. (A) FACS analysis of antiserum from FVB/N mice immunized with BL6.9 ES cells. Sera from mice before and after the second, third, and fourth immunizations were analyzed (solid line); secondary antibody only (dotted line). (B) Indicated tumor cell lines, C57BL/6 and BALB/c splenocytes, and human peripheral blood lymphocytes were stained with serum from ES cell immunized FVB/N mice (solid line), with serum from nonimmunized FVB/N mice (dotted line), and (C) with monoclonal antibody 3E8.1 (solid line) and isotype-matched antibody (dotted line).