The phenotypic expression of Hodgkin's and Reed-Sternberg (H-RS) cells was determined by analysis with a panel of monoclonal antibodies and peanut agglutinin (PNA) by an immunohistochemical technique. Seven antibodies, including T200, anti-HLA-DR, anti-Leu 10, A1G3, anti-Tac, OKT9, and anti-Leu M1, were found to react with a great majority of H-RS cells. In some cases, H-RS cells also bound PNA. Other antibodies, including those highly specific for T cells (eg, Lyt 3) and B cells (eg, B1, anti-Leu 14) were consistently negative. The results argue against the derivation of H-RS cells from T or B lymphocytes. The H-RS cells were also negatively stained with antibodies which react with monocytes (OKM1, Mo-2, 63D-3), follicular dendritic cells (DRC-1), and natural killer/killer cells (Leu 7, Leu 11a, B73.1). The presence of Leu M1 and Tac in H-RS cells is of interest. Anti-Leu M1 positivity was seen in all 20 of Hodgkin's disease (HD) cases tested and should provide a very useful reagent for differential diagnosis of HD from other reactive and neoplastic conditions. Tac normally is present only on activated T cells. The presence of Tac in H-RS cells may reflect expression of T-cell growth factor receptor or a closely related protein during a stage of neoplastic transformation. Although the nature of the neoplastic cell of HD cannot be determined by these studies, they are consistent with an origin from interdigitating reticulum cells. Both H-RS cells and interdigitating reticulum cells have a similar antigenic phenotype (Leu M1+, T200+, HLA-DR+, Leu 10+, A1G3+, and OKT9+) and a similar pattern of lysosomal enzyme activity.