The immunohistochemical localization of the alpha and beta subunits of S-100 protein in 4 cases of the malignant Schwannomas arising from von Recklinghausen's disease was investigated by the indirect immunoperoxidase method. S-100 alpha-positive cells were few in presumably normal peripheral nerves, moderate in numbers in plexiform neurofibromas and numerous in malignant Schwannomas. In contrast, S-100 beta immunoreactivity, abundantly detected in normal peripheral nerves and plexiform neurofibromas, was completely negative in all of 4 cases of malignant Schwannoma. In addition, double immunostaining method for both subunits revealed their simultaneous existence in cells in the normal nerves and neurofibroma. These results suggest that malignant change of Schwann cells convert their subunit composition of S-100 protein from beta to alpha in these malignant cells. Although the mechanisms for the proportional conversion of the subunits are as yet undetermined, the immunoreactivity of S-100 alpha subunit may be a useful marker for Schwannoma in malignancy.