The structure of an activated quaternary complex of ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) from Synechococcus PCC6301 has been solved by molecular replacement. The protein crystallizes in an orthorhombic P2(1)2(1)2(1) unit cell with a complete L(8)S(8) complex consisting of 4608 residues (37 680 non-hydrogen atoms) in the asymmetric unit. Data were collected both on film and image plate using synchrotron radiation; there were 218 276 unique reflections in the final 2.2 A data set. The eightfold non-crystallographic symmetry could be used both to improve map quality and to reduce the computing requirements of refinement. The coordinates were refined using strict non-crystallographic symmetry constraints. The stereochemistry of the final model is good, and the model has an R value of 20.0% for the reflections between 7 and 2.2 A.