Evidence supporting a broad role for the inactivation of the p53 gene in human tumorigenesis has been provided by studies showing that the p53 gene is mutated in many human cancers. In this study, we report on the mutational status of the p53 gene in prostate cancer cells and provide functional evidence that the wild-type p53 gene may have a role in suppressing prostatic tumorigenesis. Sequence analysis of exons 5-8 of the p53 gene reveals that three of five prostate cancer cell lines (TSUPr-1, PC3, DU145) contain mutations which alter the amino acid sequence of this most highly conserved portion of the gene. One of two primary prostatic cancer specimens examined also contained a mutation in this region. Transfection of the wild-type p53 gene versus a mutated p53 gene into two cell lines with p53 mutations results in reduced colony formation. Wild-type p53 gene expression is apparently incompatible with continued growth of these tumor cells inasmuch as none of the colonies which formed after wild-type transfections retain the transfected p53 sequences. Immunocytochemical data indicate that prostate carcinoma cells expressing the transfected wild-type p53 gene are growth arrested because they exhibit a reduced level of thymidine incorporation into DNA. This study is the first report of p53 gene mutations in prostate cancer cells and suggests a functional role for the p53 gene in suppressing prostatic tumorigenesis.