Hepatitis B virus–encoded X antigen contributes to the development of hepatocellular carcinoma. Given that X antigen functions by binding to other proteins, additional X‐binding proteins were sought from an adult human liver cDNA library in a yeast two‐hybrid system. The results yielded a clone encoding a 55‐kd protein that is associated with replicative senescence (p55^sen). Binding of p55^sen to X antigen was confirmed in vitro by immunoprecipitation and affinity chromatography. The expression of endogenous p55^sen inversely correlated with cell growth. Transient transfection of X antigen or p55^seninto HepG2 cells stimulated DNA synthesis by twofold to threefold, whereas cotransfection did not, suggesting that these molecules functionally interact. The detection of p55^sen in embryonic mouse liver, its absence in adult mouse and human livers, and its reappearance in livers from carriers with chronic liver disease, suggest that it may play important roles in the regulation of liver cell growth. The similarity between p55^sen and a notch ligand, which is involved in cell fate determinations during embryogenesis, implies that the binding of p55^sen by X antigen may also contribute to an alteration in cell fate, which is characteristic of carcinogenesis.