AbstractHepatitis B virus‐like particles including: small spheres and filaments 15–25 nm in diameter together with a 35–40 nm Dane particle‐like virion have been identified in sera of patients with non‐A, non‐B hepatitis. In a coded serological study, such particles were detected transiently in 3/4 acute, and persistently in 7/8 chronic cases of non‐A, non‐B hepatitis with non‐A, non‐B antigenemia. Only 2/12 similar cases without non‐A, non‐B antigens (Ag) in serum had detectable particles but neither patients with drugs, or type A hepatitis, nor cases of obstructive jaundice. The particles did not express hepatitis B surface (HBs) or non‐A, non‐B Ag at their surface but were associated, in three patients, with significant endogenous DNA polymerase activity. Furthermore, particles similar to hepatitis B cores (HBc) and also associated with DNA polymerase activity were demonstrated by sucrose gradient ultracentrifugation of a liver homogenate obtained from a patient who had died of non‐A, non‐B hepatitis. The non‐A, non‐B hepatitis virion described here appears, therefore, as a hepatitis B‐like virus. The exact kinship between these two a