The jun and fos gene families encode DNA binding proteins involved in transcriptional regulation of genes containing a TPA responsive element (TRE). To study their role in gene regulation during early mammalian development, expression and transcription regulatory properties of their gene products were investigated during retinoic acid (RA) induced differentiation of P19 embryonal carcinoma (EC) cells. Our results show, that c-jun is expressed at low but detectable levels in undifferentiated P19 EC cells and at elevated levels in its RA differentiated derivatives, corresponding with increased expression of Jun and TRE binding activity. Jun D is constitutively expressed at constant levels in both undifferentiated and differentiated P19 cells, while jun B and c-fos are not expressed. Addition of TPA to undifferentiated P19 cells does not result in induction of c-jun, jun B and c-fos, while these genes are transiently induced in RA-differentiated P19 cells. In addition, TPA treatment resulted in expression of Fos and Jun protein in RA-differentiated, but not in undifferentiated P19 cells. Addition of TPA to P19 EC cells expressing low levels of TRE binding proteins is neither followed by transcriptional activation of the TRE reporter gene nor by induction of c-jun, previously shown to be autoregulated by its own gene product. By contrast, in P19 cells differentiated by RA that contain elevated levels of TRE binding proteins, TRE dependent transcription is enhanced upon TPA treatment.