Abstract Following the intracerebral passage in 1-day-old mice of brain material from multiple sclerosis (MS) patients hydrocephalus was produced. An organism, identified as a Streptococcus, was isolated from the hydrocephalic brains. When grown in pure culture the Streptococcus was capable of producing gross hydrocephalus in 1-day-old mice by destruction of the ependyma and blockage of the aqueduct. The organism cannot be allotted to any known streptococcal species nor can it be said at this stage that it is related to MS. The failure to detect bacteria in the original MS brains by bacteriological culture methods makes its origin open to question.