Mutations of the p53 tumor suppressor gene with immunohistochemically detectable expression of p53 protein have been described in many different malignant tumors. In this study, 12 hepatoblastomas of various subtypes were investigated immunohistochemically with a monoclonal antibody for the expression of p53 protein. Immunoreactivity for p53 protein was found in both small cell tumors investigated and in embryonal areas in two out of eight tumors, but not fetal (eight tumors) or mesenchymal (four tumors) areas. The findings show that immunohistochemically detectable expression of p53 protein, which generally indicates mutation of the gene, may also be present in hepatoblastoma. The finding of p53 protein immunoreactivity in both of the small cell tumors but none of the fetal areas is consistent with a proposal in the literature concerning the histogenesis and differentiation of the various subtypes---that fetal hepatoblastoma is the most well-differentiated and small cell hepatoblastoma the least well-differentiated subtype.