The concentrations of immunoglobulins G, A, and M were measured in 62 samples of cerebrospinal fluid from 35 patients with asymptomatic neurosyphilis, 18 with symptomatic neurosyphilis, and nine with late syphilis of other organs. The most frequent and highest increase in IgG and IgA occurred in patients with symptomatic neurosyphilis. IgM was found in only five patients, but its presence, together with an increase in IgG and IgA concentrations, appears to indicate activity of the pathological process and a poor prognosis for the course of the disease.