We constructed a cDNA library against the plasma obtained from the patient with acute exacerbation of non-A, non-B liver cirrhosis, and immunoscreened this library with the sera obtained from the patients with non-A, non-B chronic liver disease. One positive clone lambda 22C containing about 1.2 kb cDNA insert was isolated from 10(6) clones. Nucleotide sequence determination and subsequent homology search revealed its identity to the tolA gene of Escherichia coli. Anti-tolA antibody was detected in 54.5% of the patients with NANB chronic liver disease whose sera were negative for antibody to hepatitis C virus (anti-C100). In contrast, anti-tolA was detected only of 14.6% patients with anti-C100 positive NANB chronic liver disease, 10.5% with hepatitis B surface antigen-positive chronic liver disease, 7.7% with alcoholic liver disease and 4.2% in normal control, and no positive case in acute hepatitis of etiology and in primary biliary cirrhosis. However, antibody to the core protein of hepatitis C virus (anti-JCC) was detected 50% of the patients whose sera were negative for anti-C100 but positive for anti-tolA. Recently, it has been reported that hepatitis C virus is rich in mutations and has some variants. These results indicated the presence of a common epitope between the tolA protein and some agent related to non-A, non-B hepatitis, especially to anti-C100 negative non-A, non-B hepatitis such as variants of hepatitis C virus which have mutations in C100 coded region.