We studied the production within the CNS of anti-HIV antibodies, of non-specific IgG, and the presence of HIV antigens in the serum and CSF of 28 HIV infected patients belonging to group IV in the Center for Disease Control classification. CSF and serum were diluted under optimal conditions to equalize their IgG content, to enable us to better interpret serum and CSF reactivity by means of Western blot and ELISA. Under these conditions, no patient displayed a limited immunological response profile in CSF as compared to serum. On the contrary, there was intrathecal synthesis (ITS) of anti HIV-antibodies in Western blot test in 21 patients for gp160 and ITS was demonstrable for env, gag, and pol products. ITS of anti-HIV antibodies occurred in 17 patients when measured by ELISA. ITS of non specific IgG and HIV-antigens in CSF were less frequent. A marked anti-HIV response is evident in the CSF-CNS compartment in the later phases of the HIV infection.