Jun and Fos, b-ZIP transcription factors, form a heterodimer and bind to DNA enhancer elements, thereby regulating the expression of target genes. The present study was undertaken to investigate the molecular mechanism underlying nuclear translocation of the Jun/Fos complex. For this purpose, normal rat kidney cells were microinjected with a DNA expression vector containing wild-type or mutant c- or v-jun together with c- or v-fos, followed by detection of the subcellular localization of Jun or Fos by immunofluorescence staining. The nuclear accumulation of Fos was markedly enhanced by the presence of wildtype Jun, but not by Jun mutants lacking nuclear targeting or zipper dimerization functions, implying that Jun and Fos mutually interact via their leucine zippers and translocate from the cytoplasm to the nucleus using the markedly stronger nuclear localization signal of Jun.