By means of a radioimmunoassay a substance excreted in feces could be detected in patients with hepatitis non-A,non-B (HNANB). Feces extracts of patients with sporadic and posttransfusion HNANB as well as of healthy persons were precipitated with PEG, digested with RNase and DNase and separated on CsCl. In HNANB-patients a RIA-positive material with a density of 1.3 g/ml CsCl could be detected which contained a partially double-stranded circular DNA. Cloning of this DNA in lambda-phase resulted in DNA of about 5 Kb, which hybridized with feces DNA under stringent conditions. The 5 Kb-DNA were mapped with different restriction enzymes. A 1.5 Kb EcoRi-fragment cross-hybridizes with HBV-DNA. No hybridization and sequence homologies were found with human, viral and procaryotic DNA as well as with plasmid and phage DNA (data base EMBL, Heidelberg). It is assumed that the DNA excreted in feces of HNANB-patients represents a viral genome not detected so far.