We have identified a new minor chlorophyll a/b-protein complex in the thylakoid membranes of spinach (Spinacia oleracea L.), which migrates as a green band below CPII on mildly denaturing polyacrylamide gels. This complex, designated CP24, was isolated from octyl glucoside/sodium dodecyl sulfate solubilized spinach grana membrane fractions by preparative gel electrophoresis and has been characterized as to its spectral properties and polypeptide composition. CP24 has a room temperature absorption maximum at 668 nanometers, a chlorophyll a/b ratio between 0.8 and 1.2, and contains three or four polypeptides between 20 and 23 kilodaltons. CP24 was also identified in grana membrane preparations from peas (Pisum sativum) and barley (Hordeum vulgare). We postulate that CP24 functions as a linker component in photosystem II, acting to orient the photosystem II light harvesting components to ensure efficient energy transfer to the reaction center.