Migration properties and occurrence of antibodies against myelin basic protein (MBP) in paired CSF and serum specimens from patients with multiple sclerosis (MS) were demonstrated after agarose isoelectric focusing, immunoblot transfer, and immunoperoxidase staining. Oligoclonal IgG antibody bands directed against MBP were found in the CSF of 9 of 28 patients with MS (32%), but not in the CSF of any of 34 patients with other neurologic diseases. No serum showed anti-MBP antibody bands. The CSF anti-MBP antibodies migrated to the anodal region of the IgG area in a different fashion from oligoclonal IgG and anti-measles IgG antibodies, which were detected in parallel. The anti-MBP bands were transient in three of seven patients whom we studied consecutively. Enzyme-linked immunosorbent assay (ELISA) of serum and CSF for detection of IgG reactivity against MBP showed absorbance values above 2 standard deviations of controls in 44% of the MS patients and in 21% of those with other neurologic diseases. Results of this assay correlated partly with those of the immunoblot assay. ELISA positive and immunoblot negative results might be due to a broad polyclonal anti-MBP antibody response.