Cerebrospinal fluid (CSF) myelin basic protein (MBP) levels, CSF/serum, albumin ratio (CSF/S alb), and 4 CSF IgG measurements-absolute CSF IgG level (CSF IgG), CSF IgG/albumin ratio, the Tibbling-Link IgG index, and the daily rate of intrathecal IgG synthesis-were measured in patients with multiple sclerosis and control subjects. In four clinical subgroups of patients, including 22 with polysymptomatic exacerbations, 22 with monosymptomatic exacerbations, 41 with chronic progressiv disease and 21 in remission, there was no correlation between CSF MBP and either CSF/S alb or the CSF IgG measurements. This finding was also observed in longitudinal studies of patients. CSF MBP lavel, as determined in a cross-sectional stydy of 325 patients with multiple sclerosis, are an exellent indicator of disease activity.