We have deduced the exon structure of the 5' untranslated region of the human cardiac alpha-myosin heavy chain gene by cloning a cDNA for this region using the polymerase chain reaction. Comparison of the cDNA and genomic DNA sequences demonstrates that the 5' non-coding region of the alpha-myosin heavy chain gene is interrupted by two introns of 645 and 337 nucleotides. Secondly we have identified the transcriptional start-site by primer extension, corroborating the previous putative assignment for the alpha-myosin heavy chain promoter based on comparisons between the rat and human genes.