Xenopus tadpole optic nerves are being used as an in vivo model system for testing the demyelinating activity of CSF and serum from multiple sclerosis patients and of sera of rabbits and guinea pigs with EAE. About 5 micron 1 of each sample is injected subcutaneously; 2-7 days later, whole mounts of the optic nerves are prepared and examined with a differential interference microscope. Counts of myelin lesions suggest that CSF from some MS patients has demyelinating activity in this system. Further tests are in progress.