To study if immunoglobulin (Ig)-secreting cells actively pass the blood-brain barrier (BBB), 15 patients with monoclonal gammopathy underwent bonemarrow (BM) iliac crest aspiration biopsy, peripheral blood (PB) sampling and cerebrospinal fluid (CSF) analysis. With an enzyme-linked immunospot assay we investigated the number and ratio of mononuclear cells secreting Ig with kappa and lambda light chains in the three different compartments. A statistically significant (P < 0.05) correlation between the ratio of Ig kappa/lambda-secreting cells in CSF, PB and BM was found. The frequency of kappa and lambda (i.e. Ig in total) secreting mononuclear cells, of the same Ig class as the paraprotein, per 10(4) mononuclear cells was higher in BM (median 2.16%, range 0.43-9.28%) compared to CSF (median 0.44%, range 0.05-9.25%) and in CSF compared to PB (median 0.12%, range 0.02-0.96%). The proportion of all mononuclear cells with Ig kappa and lambda light chain (i.e. Ig) secretion was on average 5-fold greater in CSF compared to PB and 11-fold greater in BM compared to PB. The present study indicates that paraprotein-secreting cells preferentially pass from PB to CSF.