Abstract—The proteins of spinach chloroplasts and their subfragments containing photosystem I and photosystem II, obtained by Triton X‐100 treatment or French‐pressure rupture, were separated by sodium dodecyl sulfate (SDS)‐acrylamide electrophoresis at pH 7·0 in phosphate buffer. The individual protein bands were identified where possible by comparing them with known, isolated and characterized proteins from chloroplasts, and their molecular weights were determined. The protein composition of the chloroplast fragments were correlated to the functional properties of these fragments. Distinct patterns were obtained for photosystem I and photosystem II particles. The major protein of photosystem II is expressed in the 23 kilodalton range and photosystem I proteins seem to be clustered mainly in the 50–70 kiloda