The seeds of pea (Pisum sativum L.) contain several proteins in the albumin solubility fraction that are significant components of total cotyledonary protein (5-10%) and are accumulated in developing seeds concurrently with storage-protein synthesis. One of these proteins, of low Mr and designated 'Psa LA', has been purified, characterized and sequenced. Psa LA has an Mr of 11000 and contains polypeptides of Mr 6000, suggesting that the protein molecules are dimeric. The amino acid sequence contains 54 residues, with a high content (10/54) of asparagine/aspartate. It has no inhibitory action towards trypsin or chymotrypsin, and is distinct from the inhibitors of those enzymes found in pea seeds, nor does it inhibit hog pancreatic alpha-amylase. The protein contains no methionine, but significant amounts of cysteine (four residues per polypeptide), suggesting a possible role as a sulphur storage protein. However, its sequence is not homologous with low-Mr (2S) storage proteins from castor bean (Ricinus communis) or rape (Brassica napus). Psa LA therefore represents a new type of low-Mr seed protein.