A quantitative microinjection procedure has been developed to demonstrate muscle-specific transcription of the myosin light chain 2-A (MLC2-A) promoter in differentiated chicken primary breast muscle cells. Nuclear protein binds to the distal region of the required promoter sequence but not to a mutated version of this sequence. The functional significance of this specific DNA-protein interaction for the promoter activity is demonstrated by 'in vivo' competition of microinjected MLC-CAT reporter construct together with excess of synthetic oligonucleotides encompassing the protein binding sites.