The expression of p53, a transformation associated protein, has been found to be regulated during the cell cycle. We show here that the subcellular localization of p53 varies throughout the cell cycle. In growth stimulated Balb/c 3T3 cells, p53 is produced at elevated levels and the newly synthesized protein accumulates in the cytoplasm during the G1 phase. Around the beginning of the S phase, p53 accumulates in the cell nucleus, where it stays for about 3 h. Following this initial step of DNA synthesis, p53 is no longer found in the nuclear compartment, but rather accumulates in the cytoplasm. This modulation in the subcellular localization of p53 suggests that the protein is spatially regulated during cell cycle.