A radioimmunoassay for detecting the core of the Dane particle and antibody to it was developed. The core was detected in concentrates of Dane particles but not in preparations of purified 22-nm forms of hepatitis B antigen. Detergent treatment of Dane particles increased the detectability of the core. Antibody to the core appeared following acute type B hepatitis infection and could be consistently demonstrated in chronic carriers of hepatitis B antigen. Such antibody was found less frequently in patients with evidence of re-exposure to hepatitis B antigen; it could not be detected in pooled hepatitis B immune serum globulin.