The tumor-suppressor gene p53 may transactivate the transcription of genes that down-regulate cellular growth-related genes and may become oncogenic as a result of the production of mutant proteins or the loss of its protein expression. This study reports that alterations of the highly conserved consensus intervening sequences at the splice junctions may lead to the inactivation of the p53 gene. Analyses with the combined polymerase chain reaction and single-strand conformational polymorphism and direct DNA sequencing of DNAs amplified by means of asymmetric polymerase chain reaction demonstrated sequence alterations at the splice junctions of introns 5 and 7 in four human hepatocellular carcinomas, with a single base substitution at the splice junction in three and a 10-bp deletion starting from the dinucleotide AG of the acceptor site of intron 5 in the fourth. Restriction fragment length polymorphism analysis disclosed allele loss in all three informative cases. The p53 mRNA concentrations were remarkably reduced or undetectable in two hepatocellular carcinomas, whereas the two tumors (cases 2 and 3) that had single base changes at the acceptor site of intron 7 had both normal and abnormally sized p53 mRNAs. Immunocytochemistry failed to detect the wild-type and mutant p53 proteins in all four tumors. Western-blot analysis disclosed an abnormal, larger p53 protein of 55 kD in the tumor of case 3. These findings suggest that the inactivation of p53 gene caused by the genetic alterations at the splice junction may occur more often than perceived and plays an important role in human hepatocarcinogenesis because of the inactivation of the p53 gene by way of the loss of the protein or production of an abnormal protein. (Hepatology 1994; 19:122–128).