Twenty-four homosexual adult patients suffering from LAS or AIDS were investigated for immunoglobulin class- and IgG subclass-specific antibodies to human immunodeficiency virus (HIV) by the indirect ELISA and Western blot using monoclonal tracer antibodies. All patients revealed HIV-specific serum antibodies of IgG subclass 1, and half of them IgG3. Only two had IgG2 and one IgG4 antibodies. IgM-anti-HIV was present in a person who presented a seroconversion in subsequent blood specimens. In twelve patients who developed signs of an ongoing encephalitis, cerebrospinal fluid specimens were also tested. HIV-specific IgG antibodies were usually restricted to the subclass 1. In two cases specific IgM was found to be present, although lacking in the blood specimens. By comparison with HSV antibody detection in blood and CSF, an intrathecal, possibly pathognomonic antibody formation to HIV could be confirmed, mainly directed to gp120, gp41 and p24.