Enzyme immunoassay was developed with a thermostable antigen of human brain and its corresponding antiserum of rabbit origin. Inhibition of this assay proved to be a sensitive technique for detection of this antigen in the circulation. Using the inhibition test, we tested 170 serum samples originating from 42 patients with cerebrovascular accident (CVA), 166 sera from 166 patients with other neurological disorders, and 56 sera from 56 normal subjects. Fourteen patients with CVA were found to be "positive' and 28 "negative'. Positive results in inhibition tests were first noted within 1-2 days after CVA and they appeared to decline to negative values within 3-5 weeks after CVA. None of the sera from subjects without CVA were positive.