p53 is an oncogene product which has been shown to be directly involved in malignant transformation. Furthermore, it has been proposed that this protein plays an essential role in the control of cell proliferation. In the present study, we investigated the involvement of p53 in growth regulation of cells by employing anti-sense RNA methodology to inhibit p53 expression. Transfection with p53-specific constructs spanning the entire mRNA molecule or the 5' region of the gene led to reduced p53 protein synthesis. Clones derived from such transfected cells exhibited a slower rate of DNA synthesis, as assayed by incorporation of [3H]thymidine. In most cases, transfection of plasmids encoding anti-sense RNA eventually brought about the complete cessation of cell proliferation. No such effects were observed in L12 cells, which do not synthesize p53 and therefore do not depend on p53 expression for their growth, thus excluding the possibility of a non-specific toxic effect exerted by the anti-sense p53 RNA. These findings support the notion that p53 is essential for continuous cell proliferation.