The paper describes the study of non-A-non-B virus hepatitis with parenteral mechanism of the infection transmission. Immunofluorescence method was used to test 9 liver biopsies from patients with non-A-non-B hepatitis using blood sera from convalescents after this disease. In 6 liver preparations, diffuse fluorescence of hepatocyte cytoplasm was observed. No markers of hepatitis A or hepatitis B were found. In the control group of 17 patients with HB and HA, no non-A-non-B hepatitis antigen was detected. Analysis of the blood sera from the acute period by the ELISA demonstrated the presence of anti-HBs in 28.2% of the patients with non-A-non-B virus hepatitis which corresponds to the frequency of their detection in the normal population. Antibody to HBeAg were found in 59.5%, i. e. significantly more frequently than anti-HBs in non-A-non-B virus hepatitis (p less than 0.03) and anti-HBe in the normal population (15%, p less than 0.01). Experiments of DNA-DNA hybridization demonstrated the homology of HBV DNA and that isolated from the blood serum and liver of patients with non-A-non-B virus hepatitis. The above results suggest that the agent or one of the agents of parenteral non-A-non-B virus hepatitis belongs to the group of Hepadnaviruses.