An anti-p53 ribozyme (catalytic RNA) designed to cleave the p53 pre-messenger RNA (mRNA) can efficiently reduce the level of endogenous mutant p53 mRNA. Retrovirus-mediated transduction of a hammerhead ribozyme (Rz5a) designed to cleave unspliced p53 RNA at codon 187 near the boundary of intron 5 and exon 6 reduced the level of mutant p53 RNA and protein in the human H226Br lung cancer cell line, which contains a homozygous p53 mutation at codon 254. The catalytic cleavage of the p53 pre-mRNA but not the p53 mRNA by the ribozyme was shown in vitro. The cleavage of the p53 pre-mRNA by this ribozyme was specific because a mutation in its catalytic domain (Rz5m) abolished the cleavage activity in vitro. Expression of the Rz5a ribozyme significantly suppressed the growth of the H226Br cells in culture. However, another ribozyme (Rz7a) targeted at codon 264 of the p53 gene near the boundary of intron 7 and exon 8 showed in vitro cleavage of the pre-mRNA but did not suppress cell growth. The site of modification in the p53 pre-mRNA may determine the degree of ribozyme-mediated growth suppression in this cell line. Our findings that p53 pre-mRNA can be modified by a specific ribozyme in vivo suggest a possible role for these agents in gene therapy strategies for cancer.