Thirty‐one and eleven sequences for the photosystem II reaction centre proteins D1 and D2 respectively, were compared to identify conserved single amino acid residues and regions in the sequences. Both proteins are highly conserved. One important difference is that the lumenal parts of the D1 protein are more conserved than the corresponding parts in the D2 protein. The three‐dimensional structures around the electron donors tyrosineZ and tyrosineD on the oxidizing side of photosystem II have been predicted by computer modelling using the photosynthetic reaction centre from purple bacteria as a framework. In the model the tyrosines occupy two cavities close to the lumenal surface of the membrane. They are symmetrically arranged around the primary donor P680 and the distances between the centre of the tyrosines and the closest Mg ion in P680 are around 14 A. Both tyrosineZ and tyrosineD are suggested to form a hydrogen bond with histidine 190 from the loop connecting helices C and D in the D1 and D2 proteins, respectively. The Mn cluster in the oxygen evolving complex has been localized by using known and estimated distances from the tyrosine radicals. It is suggested that a binding region for the Mn cluster is constituted by the lumenal ends of helices A and B and the loop connecting them in the D1 protein. This part of the D1 protein contains a large number of strictly conserved carboxylic acid residues and histidines which could participate in the Mn binding. There is little probability that the Mn cluster binds on the lumenal surface of the D2 protein.