This paper examines the unusual finding of delta antigen in the serum of a significant number of drug users in Dublin during 1981. The study followed a significant increase in hepatitis B among drug-users in that year. One hundred and fifty-eight cases of hepatitis B were detected among drug users in Dublin between January and August 1981 during an epidemic which began in 1980. The mean age was 21 years and males and females were equally susceptible to hepatitis B. An unusually high proportion (29%) of a sample of 50 tested had the hepatitis B associated delta antigen in their serum and confirms that the delta agent is probably common in those taking drugs intravenously.