An outbreak of hepatitis in one of Siberian regions mainly among adults of 19-35 years of age was studied. The epidemiological studies showed the infection to spread by the fecal-oral mechanism of transmission. In 90% of the patients the disease was mild, in 10% of moderate severity; no severe forms or fatal outcomes were observed. This refers also to pregnant women irrespective of the term of pregnancy. Among 78 subjects examined, antibody to hepatitis A virus of the IgM class (anti-HAV-IgM) were detected in 14%, HBsAg in 6%. No rise in titer of total anti-HAV in the convalescent period (within 3-6 months) was observed. Immune electron microscopy studies of fecal extracts from patients revealed immune complexes of virus particles 32-35 nm in size which were formed only with sera from patients in the acute stage and convalescents from this outbreak (to a lower degree). Negative results were obtained with sera containing anti-HAV and antibody to hepatitis non-A-non-B virus (HnAnB) with fecal-oral transmission mode from India, Central Asia, and Afghanistan. These data attest to the distinct nature of this causative agent among HnAnB viruses with the fecal-oral transmission mechanism.