Two enzymatically active heterodimeric forms of an aspartic proteinase, a putative 32 kDa + 16 kDa precursor form and a putative 29 kDa + 11 kDa mature form, are present in resting barley grains (Sarkkinen, P., Kalkkinen, N., Tilgmann, C., Siuro, J., Kervinen, J. & Mikola, L., 1990, in the press). The cDNA corresponding to this enzyme has been cloned and sequenced. The full-length 1863-bp cDNA sequence codes for an open reading frame of 508 amino acids. The open reading frame consists of a 66-amino acid preprosequence and a 442-amino acid mature protein. Comparison of the N-terminal amino acid sequences of the enzyme subunits with the sequence of the cDNA clone indicates that the heterodimeric enzyme is translated as a proenzyme which is processed into two subunits. The localisation of the experimentally determined N-terminal amino acid sequences of all four subunits (32 kDa + 16 kDa and 29 kDa + 11 kDa) in the same transcript, as well as the detection of only one 2.0-kb mRNA on Northern blots from resting seeds, clearly indicates that the larger (32 kDa + 16 kDa) enzyme is an intermediate precursor form of the smaller (29 kDa + 11 kDa) enzyme. The processing pattern of the barley enzyme, which is the first sequenced plant aspartic proteinase, differs from that of all other known aspartic proteinases. The barley enzyme is highly similar to mammalian and yeast aspartic proteinases, especially to human and porcine cathepsin D. This similarity is clearly dispersed over two regions, separated by a dissimilar, barley-specific region of 104 amino acids.