Delta-like protein (DLK) is a membrane protein with mostly unknown function. It is expressed by several embryonic tissues among others by the hepatoblasts of rodent and human fetal livers. We have investigated in the present study if this protein is expressed in human hepatoblastomas. The presence of DLK has been studied by standard immunohistochemistry in 31 hepatoblastomas and in several differential diagnostically related tumours: hepatocellular carcinomas and in undifferentiated childhood neoplasms. All the hepatoblastomas were positive for DLK; the surrounding liver tissue remained negative. The reaction was present in the epithelial component of the tumours. The staining pattern was mostly membranous, occasionally cytoplasmic. The other studied tumours were negative for DLK, except one hepatocellular carcinoma and the differentiating cells of two ganglioneuroblastomas. Therefore, DLK seems to be a highly sensitive and specific marker for hepatoblastomas.