We have analyzed the antiproliferative activity of the p53 tumor suppressor gene in human tumor cell lines harboring activated ras genes. The levels of p53 protein and incorporation of bromodeoxyuridine in transiently transfected cells were determined simultaneously by flow cytometry. The human HT1080 fibrosarcoma, EJ bladder carcinoma, and SW480 colon carcinoma cell lines were equally sensitive toward wild-type p53-mediated inhibition of DNA synthesis, independent of the state of the endogenous p53 protein. Overexpression of p53 genes mutated at amino acid codon 143 resulted in increased proliferation of SW480 cells, which have two mutated endogenous p53 alleles. To mimic the genetic constitution of an evolving tumor cell that has sustained a mutation in one p53 allele, we coexpressed both wild-type and mutant p53 genes controlled by strong viral promoters in HT1080 cells. Transiently transfected cells showed a reduced bromodeoxyuridine uptake similar to cells into which only wild-type p53 had been introduced. The wild-type p53 gene is a dominant growth suppressor over the mutant in all three different cell lines analyzed. By immunoprecipitation with antibodies PAb 122, PAb 420, and PAb 1620, we demonstrate the presence of both the mutant and wild-type conformations of the p53 protein in the transfected cells.