Fifty-nine multiple sclerosis patients were investigated with regard to blood-brain barrier function by determining the CSF-protein and the CSF/S albumin ratio. Abnormal values were found in 19% and 32% respectively. The occurrence of CSF-IgG elevation due to synthesis within the CNS was analysed by determining the CSF IgG/protein and CSF IgG/albumin ratios and the IgG-index = (CSF/S IgG ratio)/(CSF/S albumin ratio). Increased values were found in 75%, 80% and 86%, respectively. Oligoclonal IgG was found in CSF by agarose gel electrophoresis in 88% of the patients. The IgG-index is the parameter recommended for the demonstration of an IgG elevation in CSF due to synthesis within the CNS.