We have identified a region of human p53 protein with striking homology to a sequence motif on Simian Virus 40 T antigen which includes the nuclear localisation signal. Mutation of basic amino acid residues in this region of p53 (residues 312 to 323; SSSPQPKKKP) compromises transport of p53 protein to the nucleus. The sequence functions efficiently as a nuclear localisation signal when fused to E. coli beta galactosidase. Serine 315 within this p53 structural motif is phosphorylated in vitro by the cell cycle kinase p34cdc2. Thus in both T antigen and p53, nuclear localisation signal and p34cdc2 kinase acceptor residue map to a contiguous region of primary amino acid sequence.