As an alternative to morphological identification of circulating neoplastic lymphocytes in the blood of patients with non-Hodgkin's lymphoma, DNA of peripheral blood lymphocytes was analysed for clonal immunoglobulin gene rearrangements in 29 patients with low-grade B-cell lymphomas. 76% of the patients showed clonal rearrangements of immunoglobulin genes in their blood, including 58% of those with no other evidence of disease. In seven patients from whom paired samples were available the rearranged bands found in the blood and in lymph-node biopsy specimens containing histologically confirmed lymphoma were identical. Detection of circulating lymphoma cells by use of tumour-specific anti-idiotype antibodies and cytofluorimetry showed complete agreement with the results of DNA analysis.