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A gene encoding the variable regions of the heavy and light chains of a mouse monoclonal antibody designated H2, which specifically reacts with human liver cells, was cloned into a phagemid vector. The clone of the variable region was designed to be expressed as a separate protein, the structure of which is the same as that of the mouse antibody. The cloned phage protein specifically reacted with anti-idiotypic antibodies produced in rabbits against the original mouse antibody, and this reaction was specifically blocked by the original antibody. The soluble protein, expressed as a fusion protein, was detected as a single 30-kd band on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and specifically bound to an anti-H2 idiotypic antibody as determined by Western blot analysis. Sera of patients with various diseases were assayed for antibodies to anti-H2 by sandwich enzyme-linked immunosorbent assay (ELISA). Only sera from patients with chronic liver disease reacted strongly. This binding was specifically blocked by the cloned soluble protein. The nucleotide sequences of the variable regions were determined by the dideoxy chain-termination method, and the sequences were approximately 95% identical to those of other mouse immunoglobulins. These findings suggest that a human antibody with the same idiotype as a mouse monoclonal antibody that reacts with human liver cells, can be detected in patients with chronic liver disease, suggesting that autoimmunity may be partly responsible for these diseases.