X-ray diffraction and electron microscopy are being applied to the study of multisubunit enzymes, including aldolase from rabbit muscle, glutamine synthetase from E. coli, and ribulose-1,5-diphosphate carboxylase (RuDPCase) from tobacco leaves. The structure of aldolase is being solved by the traditional methods of isomorphous replacement. New crystal forms of glutamine synthetase are being grown and characterized. The subunit structure of RuDPCase is being studied by low resolution x-ray diffraction and electron microscopy.