Lymphocyte subpopulations and macrophages in 16 spleens from patients with Hodgkin's disease were analysed immunohistochemically using monoclonal antibodies of the Leu and the Ki M series. In non-involved splenic tissue there is an increase of T-lymphocytes with an increased T-helper/T-suppressor cell ratio, while Ki M-1, -2, -3 and -5-positive, i.e. phenotypically different macrophages, are reduced. These results indicate that involvement of the spleen in Hodgkin's disease is accompanied by changes with respect not only to lymphocyte subpopulations but also to cells of the mononuclear phagocyte system. The immunodeficiency associated with Hodgkin's disease is probably not solely due to lymphocyte dysfunction, since the disease may lead, at least in the spleen, to alterations in macrophages and accessory cells and this may contribute to the impairment of cell-mediated immunity.