Outside the central nervous system S-100 is found only in Schwann cells and satellite cells of ganglia. It has also been demonstrated in Schwannomas and neurofibromas but is absent from soft tissue tumors of non-neural origin. S-100 protein was looked for in granular cell myoblastomas using an immunohistochemical technique in an attempt to further elucidate the histogenesis of these tumors. All tumor cells in the ten tumors studied were intensely stained with antiserum to S-100 including one with some malignant features. These results support the idea that granular cell myoblastomas arise from Schwann cells.