The examination of clinical material from patients with acute hepatitis and chronic liver disease is described. Three structures are present in serum containing the hepatitis‐associated (Australia) antigen. Small pleomorphic spherical particles measuring an average of 20 nm in diameter, long tubular froms often in bizarre shapes with a diameter ranging between 20–40 nm, and larger spheroidal particles with a diameter of 40–45 nm are usually present. Coronavirus and coronavirus‐paramyxovirus‐like particles have been observed in some cases of acute and chronic hepatitis. Pleomorphic spherical particles measuring 20–23 nm have been visualized by the electron microscope in Detroit‐6 cells inoculated with the AR‐17 agent (haemovirus), and reference is also made to other studies u