Myasthenia gravis and focal polymyositis occurred in a dog following successful transplantation of DLA-identical fetal liver hematopoietic cells. There was no evidence of acute or chronic graft-versus-host disease. Antibodies to acetylcholinesterase receptor and immune complexes reactive with myoneural junctions were demonstrated, as well as focal inflammation with perifascicular and type 2 muscle atrophy. The dog responded to treatment with prednisolone and pyridostigmine.