Peripheral blood and bone marrow mononuclear cells from 10 patients with newly diagnosed Waldenström's macroglobulinaemia were analysed for the presence of leucocyte differentiation antigens using flow cytometry. Most patients had circulating intracytoplasmic-mu cells that were positive for the B-cell specific antigens B1, B2 and B4 and for the plasma cell antigen PCA-1. When such monoclonal cyto-mu + cells were purified by immune rosette depletion they were shown to express B1 to a much lesser extent than B2, B4 and PCA-1. In bone marrow samples cells with a similar phenotype, but with higher B1 expression were found in all patients. When the serum IgM paraprotein concentrations were compared to surface markers longitudinally from presentation and during the first courses of chemotherapy, a decreased serum IgM level was accompanied by a decrease in B1 cells in peripheral blood. These data demonstrate that WM cells possess a unique phenotype with coexpression of B-cell and plasma cell associated antigens.