The product of the proto-oncogene fos is a nuclear phosphoprotein. We have investigated if nuclear location of Fos protein is mediated by a nuclear targeting sequence. We show that the cytoplasmic chicken pyruvate kinase protein translocates to the nucleus if fused to the 22 amino acid Fos basic region (amino acids 139-161). However, the nuclear localization of the Fos protein is not affected if the basic region is deleted. We propose that Fos protein contains multiple nuclear targeting sequences which may act independently or in concert with one another.