Fibronectin is a glycoprotein found in plasma (cold-insoluble globulin), connective tissues, and cultures of fibroblasts and astroglial cells. This paper describes the identification of fibronectin in human CSF. Fibronectin in CSF was immunologically indistinguishable from the plasma form, as shown by double-diffusion analysis and by radioimmunoassay specific for fibronectin. Fibronectin was isolated from human CSF by affinity chromatography on Sepharose-coupled gelatin and was further analyzed by SDS-polyacrylamide gel electrophoresis. It showed a polypeptide band similar to that of plasma fibronectin. The fibronectin concentration in CSF of 17 neurological outpatients without demonstrable organic lesion in the CNS was 3.0 +/- 1.6 microgram/ml (mean +/- S.D.) which is about 0.6% of total CSF protein. In CSF of 11 MS patients, the concentration was significantly (p less than 0.005) lower (1.6 +/- 0.2 microgram/ml). Of patients with brain tumors, seven had very low levels, three were normal, and two had very high levels. The cause for the low levels in MS and tumor patients is not known.