41 patients with non-Hodgkin's lymphomas were analysed to determine occurrence of B-cell monoclonality in peripheral blood mononuclear cells using two different methods: determination of kappa:lambda ratio by light microscopic immunofluorescence, and heavy-chain gene rearrangement by DNA-technique. In 21 patients (51%) clonal heavy-chain rearrangement was found in blood, whilst 18 of the patients (44%) showed and abnormal kappa:lambda ratio. Discordant results between the methods were observed in 5 cases. Clones with gene rearrangements suggesting blood involvement were found in 16/25 (64%) patients with low grade lymphomas, in 5/16 (31%) patients with high grade lymphoma, in 17/21 (81%) patients with bone marrow involvement, in 20/27 (74%) of stage III-IV lymphomas and in all of the 14 patients with a high lymphocyte count (greater than or equal to 5.0 X 10(9]. The conclusion was that clonal analysis by the DNA-technique is a more sensitive method than the kappa:lambda determination using immunofluorescence. Even though the method is time-consuming, it could prove to be valuable in selected cases.