We obtained a monoclonal antibody that has restricted reactivity with tumor cells [Hodgkin's mononuclear cells and Reed-Sternberg (H-RS) cells] in Hodgkin's disease by immunizing mice with 12-O-tetradecanoyl phorbol-13-acetate-induced H-RS cells. The antibody, anti-IRac, reacted with H-RS cells in 8 of 20 patients who had Hodgkin's disease, as well as with interdigitating reticulum cells in dermatopathic lymph nodes and with cells of three H-RS cell lines, HDLM-1, L428, and KM-H2. The antigen IRac is a protein of molecular weight 70,000 which we found to have the following properties. (a) After 12-O-tetradecanoyl phorbol-13-acetate induction, the expression of IRac was decreased slightly in HDLM and L428 cells but increased in KM-H2. This is in contrast to a rapid decrease in the expression of two other H-RS-cell-associated antigens, CD30 and 2H9, in all 12-O-tetradecanoyl phorbol-13-acetate-treated H-RS cells. Thus, IRac may be associated with H-RS cells at advanced stages of differentiation, and its expression may not be attributable solely to cellular proliferation. (b) IRac was detected rarely in normal or in antigen- or mitogen-activated lymphocytes but was observed frequently in virus-transformed B- or T-lymphocytes. These findings were similar to those with CD30 and 2H9, indicating that the expression of all three of these antigens is probably under a similar regulatory control. (c) IRac was absent from cells in most non-Hodgkin's lymphomas; its expression could not be modulated by treatment of cells with anti-IRac. We conclude that use of IRac could facilitate the diagnosis of Hodgkin's disease and that it may be suitable for immunotherapy or immunoimaging. The expression of IRac in both H-RS cells and interdigitating reticulum cells, along with earlier evidence, indicates that H-RS cells have antigenic and functional similarities to interdigitating reticulum cells or to cells of interdigitating reticulum cell/histiocyte lineage.