This article presented the findings of a study on the serology of delta hepatitis and the detection of IgM anti-hepatitis D by EIA using serum-derived delta antigen. Research was conducted by means of a retrospective and prospective study of an outbreak of hepatitis B. This study located 135 hepatitis B surface antigen (HBsAg)positive drug misusers with acute hepatitis, and 18 HBsAg carriers attending various hospitals and clinics in Dublin, who were found to be infected with hepatitis D. The authors concluded that the findings of their research suggested that HDAg was singly the best marker of hepatitis D co-infection and, where only late specimens were available, IgM anti-hepatitis D was also useful. IgM anti-hepatitis D was also found to distinguish between chronic hepatitis D virus infection and previous exposure.