Some steps in the isolation method of photosystem 2 (PS2)-enriched partícles (BBY) influenced the Cu content of the finál preparation. In particular, the centrifugation at 10 000 X g ušed to remove starch after Triton X-100 treatment of the thylakoids, yielded starch-free BBY with a low copper content. This contrasted with the high Cu content of the starch-containing BBY. Differences in Cu levels of both preparations seemed thus to be related to the starch content of the sample. Four imidentifíed proteins were found in the starch fraction. They are probably new copper binding sites in the photosynthetic cell.