The detection of intrathecal antibody synthesis by qualitative methods or the Antibody-Index (AI) is a relevant tool for diagnosis of inflammatory neurological diseases. An increased AI can be observed for a causative antigen as well as part of a polyspecific immune response. The quantitation of the intrathecal antibody fraction in cerebrospinal fluid (CSF), F(S), helps to discriminate both cases. In contrast to AI, F(S) needs an absolute antibody concentration detected in the ELISA in mg/L. The intrathecally synthesized, "local" antibody concentration in CSF (AB(Loc)) is expressed as the specific fraction of the intrathecally synthesized total IgG (IgG(Loc)) in CSF with F(S)=AB(Loc)/IgG(Loc) x 100 in %. F(S) for HSV or measles has about 20- to 60-fold higher values in virus-caused antibody synthesis in acute herpes simplex encephalitis (mean HSV-F(S)=8.9%) or subacute sclerosing panencephalitis (mean measles-F(S)=18.8%) compared to the polyspecific immune response against these antigens e.g., in multiple sclerosis (0.14% or 0.52%, correspondingly). F(S) helps also to avoid misinterpretations of an increasing AI in cases of therapy control, and allows direct comparison of relative antibody concentrations (R(S)) in blood and intrathecally synthesized fractions in CSF (F(S)): In multiple sclerosis patients F(S):R(S) has a mean ratio of about 3 for the measles, rubella and VZV antibodies. Together with the large variability we find by ranking that about two third of MS patients have no direct correlation of the relative concentrations in serum and intrathecal synthesis. So this concept gains increasingly relevance for analysis of the polyspecific immune response in brain.