The immunohistochemical distribution and localization of the alpha and beta subunits of S-100 protein in human neoplasms and normal tissues were studied by the PAP method using monospecific rabbit antibodies against each subunit. Beta subunit immunoreactivity was detected in all S-100-positive cells and tumors reported previously. In contrast alpha subunit immunoreactivity was absent from Schwann cells, schwannomas, neurofibromas, granular cell myoblastomas, pituicytes of the neurohypophysis, Langerhans cells, interdigitating reticulum cells, and histiocytosis X cells. Interestingly, only the alpha subunit was detected in neurons of both central and peripheral nervous system, and in lymph node macrophages. Human S-100-positive cells are divided into three groups; the first is composed of cells containing only the beta subunit (probably S-100b; beta beta), the second consists of cells containing both the alpha and beta subunits, and the third is composed of cells containing only the alpha subunit (probably S- 100ao ; alpha alpha). The ontogentic relationships between S-100-positive cells and tumors are discussed in the light of these findings.