The cerebrospinal fluid (CSF) and blood of patients with multiple sclerosis (MS) were studied with respect to the frequency of lymphocytes with intra‐cellular immunoglobulins of different Ig classes as well as the relative frequency of B and T lymphocytes. An increased number of Ig‐positive cells were found in CSF (mean, 0.52%) when compared with blood (mean, 0.18%). In CSF there was a striking dominance of IgG‐positive cells, very few IgA‐positive cells, and almost no IgM‐positive cells. The distribution in blood was approximately normal. The ratios between x⁻ and λ‐positive cells in CSF were all outside the range in blood. In CSF there were fewer B cells (mean, 4.7%) and more T cells (mean, 74.2%) when compared with blood (mean, 11.5% and 61.8%, respectively). The values for MS blood were approximately the same as for normal controls. The increased number of IgG‐containing cells in the CSF are in agreement with earlier studies, which showed a local immunoglobulin synthesis. The increased proportion of T lymphocytes in CSF of MS patients may indicate that these cells play a role in the pathogenesis of MS.