Peripheral blood lymphocytes from 96 patients with non-Hodgkin's lymphoma were studied, either at primary staging, during treatment or in follow up. The amount of surface immunoglobulin light chain per cell was determined by direct immunofluorescence staining analysed by flow cytometry. Discrepancy between kappa and lambda fluorescence profiles in the sample was considered to indicate the presence of monoclonal cells i.e., circulating lymphoma cells. The results were correlated with routine haematological findings, histopathology of the lymphoma and tumour burden. Using routine haematological methods leukaemic spread was evident in 24% of the patients in our study. Using kappa/lambda distribution analysis evidence of circulating lymphoma cells was found in an additional 27%. As expected, the major diagnostic gain was in the low grade malignant group, where 30% of the patients with normal peripheral blood according to standard procedures showed evidence of circulating lymphoma cells in the kappa/lambda distribution analysis. The corresponding gain in the high grade malignant group was 19%. In patients with active disease but without morphological evidence of leukaemia, 37% showed abnormal kappa/lambda distributions. In patients in complete remission the corresponding figure was 18%. The clinical significance of small numbers of circulating lymphoma cells is not yet understood, but a possible outlook is to use kappa/lambda distribution analysis to increase staging precision and in the early detection of relapse.