A protein complex containing chlorophyll a/b was isolated from a photosystem I complex. The protein has a single polypeptide with a molecular weight of 20 kilodaltons and a chlorophyll a/b ratio of 3.5 to 4.0. The complex absorbs light most strongly at 667 nanometers and emits fluorescence at 681 nanometers. It was isolated using gel electrophoresis and chromatography techniques. Antibody tests showed that this chlorophyll a/b protein complex is distinct from other chlorophyll-containing proteins in photosystems I and II.