In the human lymphoreticular system, the alpha and beta subunits of S-100 protein are found in ordinary monocyte-macrophages and non-phagocytic histiocytes such as Langerhans cells and interdigitating reticulum cells, respectively. The beta subunit is also present in some CD8+ T cells. In the present study, we investigated the ontogeny of these histiocytes and lymphocytes in humans. Yolk sacs and 4 to 21-week fetuses were examined immunohistochemically for the presence of S-100 protein subunits using antisera monospecific to each subunit. S-100 alpha + macrophages were present in the yolk sacs and the hepatic sinusoids of the 4th week embryos prior to bone marrow hematopoiesis. These macrophages later appeared in other lymphoid organs when anlagen of these organs were formed. No S-100 beta + cells were found in the yolk sacs. S-100 beta+ histiocytes were first detected in the hepatic sinusoids of the 5th week embryo, and after the 8th week of gestation, they were distributed in other lymphoid organs. S-100 beta+ lymphocytes were not found in the liver. They were first detected in the thymus at the 12th week of gestation, and were subsequently distributed in other lymphoid organs. These results suggest that S-100 beta+ lymphocytes and histiocytes may belong to different cell lineages, and the former may not be the precursor of the latter.</p
