Reaction centers have been crystallized from the antenna-deficient RCO2 strain of Rhodobacter sphaeroides, and a structural model has been constructed at 2.6 Å resolution. The antenna-deficient strain allows assessment of the structural integrity of the reaction center at each stage in the purification crystallization procedure. Spectroscopic evidence indicates that the properties of the reaction center bacteriopheophytins and the primary donor bacteriochlorophylls are modified somewhat on removal of the protein complex from the membrane and that these changes are carried through to the crystal form of the reaction center. The structure of a FM197R/YM177F mutant reaction center has also been determined to 2.55 Å resolution. The mutant complex shows an unexpected change in structure, with a significant reorientation of the new arginine, the incorporation of a new water molecule into the structure, and rotation of the 2-acetyl carbonyl group of one of the primary donor bacteriochlorophylls to a more out-of- plane geometry. Changes in the optical spectrum of the FM197R/YM177F reaction center are discussed with respect to the altered structure of the complex.