A single-step solid phase radioimmunoassay was developed to detect human carbonic anhydrase (CA) isoenzymes I (CA I) and II (CA II) in cerebrospinal fluid (CSF). The assay is capable of routinely detecting both isoenzymes at ng levels compared to the microgram levels of the traditional catalytic methods, which failed to demonstrate any CA activity in CSF. When the values of immunoreactive CA II in CSF were corrected for blood contamination (the CA I/CA II ratio of blood was about 7.9), the amount of brain tissue originated CA II could be calculated. The CA II values in CSF samples from 13 patients with multiple sclerosis were higher than those in CSF samples from 11 patients with various peripheral neurological disorders. Since CA II has been specifically localized to oligodendrocytes and myelin, our preliminary results suggest the possibility of CA II leakage from oligodendrocytes and myelin into CSF in demyelinating disease.