A new monoclonal antibody (mAb), 1E8 (IgG1, kappa), was obtained from a hybridoma prepared by fusion of mouse myeloma cells (NS-1) with splenic cells of mice immunized with a human B blastic malignant lymphoma cell line, HPE-Ret-3 (Ret-3). The mAb showed a reactivity unrestricted to a specific cell lineage on flow cytometrical analysis of the reactivity with human lympho-hematopoietic cell lines. In peripheral blood, 1E8 reacted with the cells of all lineage, that is, lymphocytes, monocytes, granulocytes and platelets, even though its intensity was very low by immunohistochemistry. Immunohistochemical examination of human tonsil with 1E8 showed a characteristic staining pattern. Positive cells scattered in follicular (mantle zone and germinal center), parafollicular (T-dependent area), subepithelial and interstitial connective tissue areas. These positive cells seemed to be categorized into dendritically shaped cells (DSC), including dendritic cells (DC) and a subpopulation of macrophages in follicles, interdigitating cells (IDC) and irregularly shaped mononuclear cells. The localization of 1E8 antigen staining was similar to that of integrin CD11c, although its distribution on hematopoietic cell lines did not coincide with that of 1E8 antigen. Immunobiochemical studies showed that 1E8 bound two cell surface proteins with molecular size of 70,000-90,000 and 35,000 Da each. Consequently, 1E8 antigen might be a novel marker of DSC.