The cerebrospinal fluid (CSF)/serum concentration quotients of albumin and the immunoglobulins G, A and M have been determined for 396 patients with a normal blood CSF barrier function or a pure barrier impairment without a humoral immune response within the central nervous system. The ratios between the concentration quotients of the three immunoglobulins and that of albumin increase in a nonlinear fashion with progressive impairments of the blood CSF barrier, ie the molecular size dependent discrimination of the protein transfer (‘selectivity’) decreases. The upper reference values of the concentration quotients for the whole range of passive transfer comply with the hyperbolic function: Q (IgX)= a b√ Q (Alb) 2+ b 2− c with different constants a b, b 2, c for IgG (0.8, 15× 10− 6, 1.8× 10− 3), IgA (0.72, 80× 10− 6, 5.1× 10− 3) and IgM (0.65, 150× 10− 6, 7.5× 10− 3). With these discriminatory functions the locally synthesized fractions IgX (loc) of IgG, IgA and IgM in CSF can be calculated by the general formula: IgX (loc)=[Q (IgX)− a b√ Q (Alb) 2+ b 2+ c]. IgX (Ser).