The cDNA coding for the signal peptide of rat liver mitochondrial aldehyde dehydrogenase was sequenced. The deduced amino acid sequence of the signal peptide was MLRAALSTARRGPRLSRLL. From this sequence an amphiphilic helix which had a high hydrophobic moment could be constructed. A comparison to the published cDNA sequence of human mitochondrial aldehyde dehydrogenase revealed great sequence identity and allowed us to make some predictions regarding the primary structure of the human signal peptide.