An 85-kDa antigen was detected in cerebrospinal fluid (CSF) using a monoclonal antibody, 11.57, raised against a fraction of paired helical filaments extracted from Alzheimer’s disease (AD) brain tissues. The antigen is a thermostable protein that is unrelated either to tau or amyloid β-protein precursor. Its level was measured in lumbar and post-mortem CSF samples and found to be decreased significantly in patients with dementia as compared with non-demented controls. There was no difference in its level between patients with AD and those with non-Alzheimer type dementia. In AD, however, there was a significant relationship between disease progression and the amount of the 85-kDa antigen.