• A micromethod to detect oligoclonal IgG from 50 μL of unconcentrated CSF was developed by using polyacrylamide gel electrophoresis in sodium dodecyl sulfate (SDS-PAGE). Of 17 patients with multiple sclerosis, oligoclonal bands were demonstrated in 16 instances (94%) by micro-SDS-PAGE and in 13 (76%) by agarose gel electrophoresis. The corresponding figures among 30 patients with optic neuritis were 16 (54%) and five (17%), respectively, and among ten patients with other neurological disease the figures were two (20%) and none, respectively. Thus, micro-SDS-PAGE is more sensitive than agarose gel electrophoresis for detection of oligoclonal IgG. The small volume of unconcentrated CSF that is required enhances the usefulness of this test.