An empirical formula has been developed to calculate the de novo rate of synthesis of IgG in the central nervous system (CNS), based on physiologic principles that govern the passage of albumin and IgG across the blood-brain barrier (BBB). To validate the formula, radiolabeled IgG and albumin from pooled normal t he followed from the blood to the cerebrospinal fluid (CSF) over 21 days in nine patients with definite multiple sclerosis (MS). IgG synthesis rates were calculated by the isotope exchange method and compared to values obtained with the empirical formula. There was excellent concordance, from a low rate of synthesis of 5 mg per day to a high rate of 120 mg per day. A double radiolabeled IgG experiment in two patients showed that the MS BBB processed normal serum IgG in the same way as IgG derived from autologous MS serum. Accordingly, the empirical formula, which requires only one sample of CSF and matched serum, can reliably and validly estimate the de novo rate of IgG synthesis in CNS of patients with MS.