The hepatitis B virus-associated delta antigen was detected in the serum of a young female drug addict with acute hepatitis and a previous history of hepatitis B surface antigen carrier. Delta antigen was associated with a 35- to 37-nm subpopulation of hepatitis B surface antigen particles which banded at a density of 1.25 gm per cm3 of CsCl and contained a small RNA of approximately 5.5 X 10(5) molecular weight. This report demonstrates that delta antigen is associated with a unique subpopulation of hepatitis B surface antigen particles and small RNA in human serum.