Using a new ELISA method we have measured the IgM concentration in the serum and the cerebrospinal fluid CSF from 110 neurological patients. Among there, 41 had multiple sclerosis (MS), 48 other inflammatory diseases (OID), including 30 AIDS, and 21 non-inflammatory neurological diseases (NID). A highly significant correlation was established between results with native IgM and the dithiothreitol reduced IgM. An intrathecal synthesis (ITS) of IgM was detected using the CSF IgM/CSF albumin ratio, the IgM index and a quantitative formula in 33 patients: nine MS, 23 OID (including 18 AIDS) and one NID. The frequency of IgM ITS was 22% in MS patients, 48% in the OID (60% in AIDS) and 5% in the NID groups. This ITS was not impaired by an increase in serum IgM concentration or by a blood-CSF barrier damage. These facts confirm that intrathecal immunity is not a "steady-state" related to the general immunity but a specific response restricted to the central nervous system. Conversely, CSF IgM increase and IgM ITS were closely related (p < 10(-6) ). In addition, IgM ITS and IgG ITS were found to be highly correlated in OID, especially in AIDS patients: such correlation was not observed in the MS group. No significant correlations were observed between IgM ITS and any of the clinical parameters in MS patients. These results suggest the probable specificity of IgM ITS in MS patients.