Simultaneous serum and CSF Ig[immunoglobulin]M determinations were made by electroimmunodiffusion in 5 normals and 203 patients. Absent from normal CSF, IgM was found in 56 patients (27%). A blood-CSF transudation was present in 25 patients (12%). IgM seems of no diagnostic value in these cases. A purely local synthesis was demonstrated in 17 patients (8%), concerning proved (bacterial, viral, parasitic) or supposed (multiple sclerosis) persistent infectious aggressions. In the last 14 patients with IgM in CSF, a transudation was associated with a local synthesis of IgG (meningitic pattern). A ratio for IgM .cntdot. 103/albumin above 10 seems to be an indicator for multiple sclerosis.