An immunoperoxidase technique for the detection of carcinoembryonic antigen was applied to 10 cases of granular cell myoblastoma. Consistent, strong, intracytoplasmic granular staining, which can be easily interpreted, was obtained in all cases. Schwannomas, neurofibromas, dermatofibromas, and leiomyomas were negative. The test is helpful in confirming doubtful cases. The results tend to support the suggestion that granular cell myoblastoma is derived from perineural rather than endoneural cells.