The fluorescent treponemal antibody test for cerebrospinal fluid (FTA-CSF) using monospecific conjugates anti-IgG, IgM, and IgA was used to determine the presence of anti-treponemal antibodies in the spinal fluid of 335 patients with primary, secondary, and latent syphilis and symptomatic and asymptomatic neurosyphilis and of patients with certain neurological disorders. Of these, 230 (68·65%) patients had non-reactive results to this test. Of the remaining 105 patients, 78, 63, and 10 had reactive results with anti-IgG, IgM, and IgA conjugates respectively. Of the 129 cases of known syphilis, 11 were diagnosed as primary, 32 as secondary, and 50 as latent, and 36 patients had neurosyphilis. None of the specimens from the patients with primary syphilis gave reactive results to the test. Specimens from 21 (65·62%) of the 32 patients with secondary syphilis, 30 (60%) of the 50 patients with latent syphilis, and all (97·22%), except one, of the 36 patients with neurosyphilis gave reactive results to one at least of the IgG, IgM, or IgA FTA-CSF tests. Among the specimens from patients with secondary syphilis twice as many gave reactive results with anti-IgG conjugate than with anti-IgM conjugate. However, with specimens from patients with latent syphilis and neurosyphilis this ratio was diminished to 1·5:1. The Kolmer complement-fixation test, although superior in sensitivity and specificity to the Venereal Disease Research Laboratory (VDRL) test, in patients with secondary and latent syphilis and neurosyphilis, was greatly inferior to the FTA-CSF test. Data indicate that anti-treponemal antibodies can be detected in the spinal fluid even in patients with no neurological symptoms in cases of secondary syphilis and that the FTA-CSF test can be a valuable tool in the early detection of an immunological response to treponemal infection in the spinal fluid.