The presence or absence of large protein molecules of plasma type in the cerebrospinal fluid (CSF), e.g., fibrinogen, may be of diagnostic interest on examination of the passage of proteins from the blood to the CSF for any changes suggestive of neurological disease. It is well known that a fibrin web may develop when the CSF from patients with tuberculous meningitis is left to stand for some hours at room temperature. In such samples the total protein content is high. According to Lups and Haan4 fibrinogen does not normally occur in the CSF. This would imply that the presence of fibrinogen in the CSF could be taken as a sign of derangement of the passage of proteins from the blood into the CSF, and thus of neurological disease. In human plasma, fibrinogen is present in a concentration of about 0.3 gm. per 100 ml. If present in the CSF