Observations of 416 patients with and 112 convalescents after non-A-non-B virus hepatitis (HnAnB) with fecal-oral mechanism of transmission were carried out in 1984-1986. An enzyme immunoassay (EIA) was developed for the detection of HnAnB antigen in faeces. The rate of this antigen detection varied from 9.1% to 34.6% in different areas. The HnAnB antigen is present in two areas of cesium chloride density gradient: the maximum at 1.39-1.40 g/cm3 and another peak in the zone of 1.18-1.23 g/cm3. The method of radioimmunoprecipitation followed by SDS-polyacrylamide gel analysis showed the presence of several polypeptides of HnAnB virus. Most clearly, protein 30 K was documented, also polypeptides 91-94 K and 48-57 K were demonstrated. The resistance of nucleic acid of HnAnB virus to RNase was tested. The results indicate that the isolated NA is likely to be DNA. The above data suggest that the agent inducing HnAnB belongs to the group of parvoviruses.