A mouse monoclonal antibody directed against the protein product of the hepatitis B virus X open reading frame was prepared. This antibody was used to screen liver tissue sections from patients with chronic hepatitis (CH) and patients with liver cell carcinoma (LCC). Reactive antigen was detected by immunohistochemistry in about 30% auf the samples from CH patients and in about 80% of the samples from LCC patients regardless of whether tumor or surrounding nontumor tissue was analyzed. A predominant localization of the antigen in the cytoplasm was observed. In liver sections of CH patients the presence of HBx or HBx-related protein appeared to correlate with the presence of the classical viral antigens HBs- and/or HBcAg. A similar correlation was not found in liver or tumor tissue samples from LCC patients. The occurrence of X-monoclonal-antibody-reactive protein (Xarp) at a low frequency in liver tissue from patients without hepatitis B virus related disease suggests that Xarp in some cases may not be identical with the putative viral X antigen.