The distribution of S-100 proteins was examined in 98 pediatric brain neoplasms with rabbit antiserum to S-100 protein, utilizing the peroxidase-antiperoxidase (PAP) immunohistochemical technique. Positive cytoplasmic immunoreactivity was present in 22 of 24 astrocytomas, both oligodendrogliomas, eight of 13 ependymomas, all 11 choroid plexus tumors, all nine schwannomas, two of six meningiomas, and both meningeal melanomas. Mixed cytoplasmic immunoreactivity was present in both gangliogliomas, all seven craniopharyngiomas, and the only hemangioblastoma. No cytoplasmic immunoreactivity was found in primitive neuroectodermal tumors or germinoma. Nuclear immunoreactivity was present in some cases. Because of the broad spectrum of apparent immunoreactivity with S-100 antiserum, S-100 protein cannot be used as a specific marker of glial or Schwann cell tumors.