We have measured the molecular weight of the small subunit of Fraction I protein from pea and broad bean by sodium dodecyl sulphate polyacrylamide gel electrophoresis, Sephadex gel-filtration and amino acid composition data. The results suggest a molecular weight of 12 000-14 500, although measurements by gel-filtration in alkali suggest a molecular weight of approximately 22 000. N-terminal amino acid sequence data and C-terminal determinations show that the protein consists of a single type of polypeptide chain, although the anomalously high molecular weight obtained on gel-filtration in alkali does not preclude the existence of the polypeptides as dimers under certain conditions.