Immunoglobulin G (IgG) and M (IgM) antibodies against mumps virus were measured by an enzyme-linked immunosorbent assay (ELISA) in the serum and cerebrospinal fluid (CSF) specimens of patients with mumps meningitis. The CSF IgG antibodies correlated well with the respective antibody titers in serum. On the contrary, in only about half of the patients a moderate correlation was found between the CSF and serum IgM antibody titers, while the other patients did not have detectable mumps IgM antibodies in CSF irrespective of intermediate to high titers in serum. Two different immunologic mechanisms may be involved in these two groups which, however, did not show any clinical differences. The lack of IgM antibodies in the CSF of many patients diminished the value of CSF in the laboratory diagnosis of mumps meningitis compared to use of serum specimens. Intrathecal synthesis of mumps IgG antibodies was demonstrated in 83% of the patients, and of IgM antibodies in at least 67% of those patients with detectable IgM antibodies in CSF. The ratio between mumps IgG and IgM antibodies was higher in CSF than in serum, suggesting that the synthesis of IgG antibodies in central nervous system was more efficient than that of IgM antibodies.