p53 protein was localized in the cytoplasm of growing and in the nucleus of growth-arrested MCF-7 cells. While the absolute amount and rate of synthesis of p53 in growing and arrested cells were nearly the same, the protein in growing cells was phosphorylated to a greater extent than in arrested cells. The abilities of the cytoplasmic and nuclear p53 proteins to bind to DNA sequences specific for p53 protein binding did not differ remarkably despite their differential phosphorylation levels. Serum-induced translocation of the p53 protein from the nucleus to the cytoplasm, as well as DNA and protein synthesis, were inhibited by cycloheximide. These results suggest that the DNA synthesis-associated cytoplasmic translocation of p53 protein in response to serum stimulation depends on de novo protein synthesis and not on alteration of the protein's ability to bind to specific DNA sequences.