Evidence of infection with delta agent was sought in 284 patients with acute hepatitis B referred to Fairfield Hospital, Melbourne, between July 1, 1981, and June 30, 1982 and in 127 hepatitis B surface antigen (HBsAg) carriers. Acute and convalescent serum samples from the patients with acute hepatitis B and single or multiple sera from the HBsAg carriers were tested for delta antigen (delta ag) and/or antibody (anti-delta) by solid-phase radioimmunoassay (SPRIA). Evidence of infection was detected in 14.4% of patients with acute hepatitis B and 7.1% of chronic HBsAg carriers. Eighty-eight percent of those in whom delta agent infection was demonstrated were intravenous drug users.