The value of testing for core antibody (anti HBc) in acute hepatitis was assessed in 503 patients. All hepatitis B surface antigen (HBsAg) positive patients tested were also anti HBc positive. Of the 110 HBsAg negative, anti HBc positive patients, 32 were found to have surface antibody, indicating previous infection with hepatitis B virus (HBV). Of the remaining 78 patients in whom anti HBc alone was detectable, follow-up specimens were received from 28 and, of these, 21 were then found to be anti HBc negative. Thus in acute hepatitis non-specific transient reactions to core antigen may appear, and the presence of anti HBc alone cannot be considered adequate evidence for a diagnosis of HBV infection.