In this study the antigenic profile of Hodgkin (H) and Sternberg-Reed (SR) cells from cases of Hodgkin's disease was analysed using a large panel of monoclonal and polyclonal antibodies reactive with cells of lymphoid and haemotopoietic origin. The aim of this investigation was, firstly, to throw light on the origin of H and SR cells and, secondly, to determine whether there is any evidence to support recent suggestions that H and SR cells differ antigenically between different histological categories of Hodgkin's disease. Frozen sections (from 24 cases) and paraffin sections (83 cases) were stained by immunoenzymatic methods and the results compared with those obtained from staining a wide variety of reactive and neoplastic tissue samples (including examples of tuberculosis, sarcoidosis, malignant histiocytosis, histiocytosis X, osteomyelosclerosis and non-Hodgkin's lymphoma). The results revealed that H and SR cells of all types of Hodgkin's disease consistently lack markers found on null cells, B cells, T cells, cells of monocyte/macrophage series, interdigitating reticulum cells, dendritic reticulum cells and erythropoietic and thrombopoietic cells. However, H and SR cells constantly expressed an antigen detectable with the recently produced monoclonal antibody Ki-I. The vast majority of typical and lacunar type H and SR cells contained the granulocyte-related antigens detected by monoclonal antibodies TU5, TU6, TU9 and 3C4, whereas other more or less specific granulopoietic cell markers (such as peroxidase, chloroacetate esterade, lysozyme, cationic leukocyte antigen and OKMI) were consistently absent. H and SR cells in cases of nodular paragranuloma (nodular type of Hodgkin's disease with lymphocyte predominance) were not monotypic in light chain type (as has been previously reported), but rather contained chi and lambda chains within the same cells, as do typical and lacunar type H and SR cells. Immunostaining of normal and hyperplastic lymphoid tissue with the Ki-I antibody led to the detection of a new, as yet unidentified, small-cell population of unknown origin and function, which is present between, around, and within cortical follicles. It is concluded from these findings that H and SR cells constitute a unique cell type that differs in many properties from all other known cell types. Furthermore, H and SR cells of the various histological types of Hodgkin's disease are more closely related than previously believed. It is suggested that the hitherto unknown cell population detected with the monoclonal antibody Ki-I in normal lymphoid tissue is the normal equivalent of H and SR cells.