The level of IgM was determined by Particle Counting Immunoassay in the cerebrospinal fluid. In non-neurological patients (N = 20) the mean was 97.5 micrograms/l with the upper reference limit at 380 micrograms/l. The mean IgM index was 0.021 with the upper reference limit at 0.071. Of 21 patients with stroke, 5 had an IgM index exceeding the reference limit. High levels and indices of IgM were observed in most patients (N = 27) with infectious meningo-encephalitis. In this group, the IgM index was abnormal in about 30% of cases with a normal total protein content, and was more often increased than the IgG index. In multiple sclerosis patients (N = 80), the IgM index was increased in 32%. In this disease very high values of IgM index (greater than 0.13) were never associated with very high values of IgG index (greater than 1.8). A significantly higher proportion of males was found in the group of patients with very high values of IgM index (N = 11). No significant influence of the age of onset, the interval between onset and sampling and clinical state was observed. However, of 10 patients with a multiple sclerosis history exceeding 15 years none had an IgM index exceeding the upper reference limit. Four patients with multiple sclerosis had a high IgM index without either an increase of the IgG index or the presence of oligoclonal bands.