Injection of neonatal bone marrow cells from mice lacking the gene encoding suppressor of cytokine signaling 1 (SOCS1) into irradiated syngeneic 129/Sv or C57BL/6 mice led to a decreased survival, more rapidly occurring in 129/Sv than in C57BL/6 mice. Moribund mice did not exhibit the acute or chronic diseases developed by Socs1-/- mice but developed a pathology characteristic of graft-versus-host disease with typical chronic inflammatory lesions in the liver, skin, lungs, and gut. The results indicate that cells derived from the Socs1-/- bone marrow are autoaggressive but did not identify the cell types involved. Failure of the engrafted Socs1-/- marrow cells to reproduce the tissue damage typical of Socs1-/- disease indicates that loss of SOCS1 from target tissues may also be required for the development of the Socs1-/- diseases, such as fatty degeneration of the liver, polymyositis, or corneal inflammation.