Cerebrospinal fluid lymphocytes from six human immunodeficiency virus (HIV)-infected and three seronegative patients were studied for in vitro synthesis of HIV-specific antibody by means of Western blot analysis of lymphocyte culture supernatants. All the HIV-positive subjects showed in vitro production of HIV-specific IgG, regardless of neurological involvement, while no specific antibody synthesis was detected in seronegative individuals. These data provide the first direct evidence that HIV-specific antibody in cerebrospinal fluid is due to intrathecal synthesis.