Presence of monoclonal B-cells in peripheral blood (MBCB) was studied in 132 previously untreated patients with non-Hodgkin lymphoma classified according to the Kiel classification. Detection of B-cells was performed by immunofluorescence microscopy, using antibodies against immunoglobulin light chains. Thirty-six patients (27%) were found to have MBCB. In the subgroup of low grade lymphomas 25/75 (33%) and in high grade lymphomas, 11/57 (19%) had MBCB. Presence of MBCB was correlated to clinical stage and 81% of the patients with MBCB were in stage IV. Twenty-two out of 36 (61%) patients with MBCB had normal lymphocyte counts (less than 5.0 X 10(9]. MBCB indicated a less favourable prognosis, mainly due to the close association with stage IV. It was concluded that studies of MBCB by this simple method are useful in detecting 'subclinical' blood involvement and valuable in the initial staging procedure as well as in the follow-up of the patients.