SUMMARY.Alterations in the expression of the RB tumour suppressor gene were found by Western immunoblot in three of seven hepatocellular carcinoma and hepatoblastoma cell lines. Abnormalities were detected by single‐strand conformation polymorphism (SSCP) within exons 17–21 in RB cDNA from two of these three cell lines and within exons 20–21 in the third cell line. In these three cell lines with abnormal RB expression, abnormal expression of the p53 tumour suppressor gene was also found, apparently the product of a mutant gene. Thus, mutations within the RB gene (or splice‐site mutations with exon‐skipping) and apparent mutations of the p53 gene together may have contributed to the development of three of these tumours or to the establishment of these c