Platelet-derived growth factor (PDGF) is a heterodimeric glycoprotein consisting of A and B chains. A functional promoter had been identified in the 5' flanking region of the human PDGF-A chain gene. We found that the PDGF-A chain promoter region possesses the potential for bidirectional activity. This bidirectional promoter activity is influenced by the 5' -untranslated region (5'-UTR) and serum concentration. The 5'-UTR may regulate expression of the PDGF-A chain by transcription in the opposite direction.