Peripheral blood from 69 patients with non-Hodgkin's lymphoma was examined with respect to B and T cell markers. Evidence for monoclonal B cell was found in 29 cases, 8 of 'high grade' and 21 of 'low grade' malignancy according to the Kiel classification. 17 out of the 29 patients had a normal lymphocyte count. Using conventional staging methods 4 cases of the 29 were in stages II and III, all others in stage IV. The proportion of S-phase cells in peripheral blood, determined by flow cytometry, was found to be elevated in cases with a monoclonal cell population. It is concluded that surface marker analysis of blood cells may be valuable as a diagnostic tool, as an indicator of prognosis and perhaps for the staging procedure of malignant lymphomas.