• Oligoclonal immunoglobulins (Ig) demonstrable as bands by agarose-gel electrophoresis of CSF were characterized in ten patients with aseptic meningitis. Oligoclonal CSF IgG was found in all patients, IgM was found in two. The oligoclonal Ig was more often of λ light-chain type than of k type. In four patients, oligoclonal Ig bands contained k and λ light chains simultaneously. In two patients, bands consisting of free λ light chains were found. On repeated investigations more than one year later in three cases, the oligoclonal IgG was of the same lightchain type. The immune response within the CNS in aseptic meningitis seems to be dominated by synthesis of IgG with λ light chains.