Ongoing studies are attempting to determine the origin and immunologic function of Reed-Sternberg cells in Hodgkin's disease by characterizing a neoplastic cell line obtained from a patient with advanced Hodgkin's disease. Initial studies demonstrated that the L-428 cell line is a potent stimulator of the human primary mixed lymphocyte response. The time course, dose response characteristics, nature of the responding cell, and the ability of the response to be blocked by monoclonal anti-I-A antibodies are all characteristic of mixed lymphocyte reactions. Of interest, the MLC response occurs without detectable interleukin I production in the cultures. This cell line is also capable of serving as an accessory cell for proliferative responses of purified T cells to mitogens. Purified T cells from patients with advanced stages of Hodgkin's disease have reduced proliferation in the presence of the L-428 accessory cell consistent with an inherent T cell deficit in patients with Hodgkin's disease. Studies have been initiated to determine the ability of the L-428 cells to present soluble antigens to T cell clones in a genetically restricted fashion. In regard to immunologic function and cell surface characteristics, the L-428 tumor cells resemble the dendritic cell. Mouse monoclonal antibodies have been prepared against the L-428 tumor cell and react with Reed-Sternberg cells in tissue sections obtained from patients. The specificity of these monoclonal antibodies is now being determined. The characterization of the antigen being recognized by the monoclonal antibodies is in progress.