A case of Reed-Sternberg (R-S) leukaemia is described, and the results of the morphological, cytochemical and cytokinetic studies on the circulating neoplastic cells are reported. Detailed data are given for each of the 3 types of abnormal circulating cells: abnormal mononuclear (AM) cells, Hodgkin's (H) cells and R-S cells. Our results cannot discriminate whether R-S cells derive from monocyte-macrophages or from B-cell lineage. However, some data suggest that H and R-S cells may likely originate from AM cells. The unfavourable clinical significance of the appearance of circulating R-S cells is discussed taking into account the other few cases reported in literature.