We investigated the immunohistochemical characters about structure and ontogenic development of sheath artery and ellipsoid of White Leghorn chicken spleen. We succeeded to establish two newly monoclonal antibodies, named CSA-1 and CSA-2. CSA-1 reacted not only with all vascular endothelial cells, but also with fibroblastic reticulum cells of the inner zone of ellipsoid. CSA-2 detected dendritic cells surrounding the outer zone of ellipsoid, on which class II major histocompatibility complex (MHC) were expressed. In the ontogenic study, it was shown that CSA-1-positive reticulum cells appeared on day 18th of fetal stage, which was the critical date when the tight junction of endothelial cells of sheath artery disappeared. And CSA-2-positive dendritic cells and macrophages accumulated around sheath artery on the same day 18th. Using anti-chicken class II MHC monoclonal antibody, CBL-1, macrophages and dendritic cells of ellipsoid were possitive, but endothelial cells of sheath artery were negative against CBL-1. Even with immunological activation by intravenous injection of horseradish peroxidase, endothelial cells didn't express class II MHC. Therefore, it is strongly suggested that macrophages and dendritic cells of ellipsoid, but endothelial cells, play important roles in immune response.