Previously we observed that sera from recipients of allogeneic bone marrow transplants who developed extensive chronic graft-versus-host disease (GVHD) intensively stained the nucleolar region of target cells in indirect immunofluorescence microscopy. To identify the target antigens, immunoblotting experiments were performed using isolated nuclei, isolated nucleoli, and purified nuclear and nucleolar proteins as the antigen source. The nucleolar phosphoproteins B23 and C23 were identified as the main target antigens. Eleven of 19 extensive chronic GVHD sera reacted with these nucleolar phosphoproteins. In addition, four sera recognized histone H1, and two sera recognized the nuclear lamins A and C. Our patients reacting with the nucleolar proteins had symptoms resembling that of scleroderma or Sjögren's syndrome.