We have crystallized renin from the submaxillary gland of male mice, both in its native state, and in binary complex with transition-state analog inhibitors. The best of the many crystal forms examined consisted of tetragonal bipyramids with space group symmetry P41 21 2 (or its enantiomorph) and unit cell dimensions a = b = 91.4 A, c = 211.6 A; alpha = beta = gamma = 90 degrees. This tetragonal form was compatible with both inhibited and uninhibited renin. Two of the inhibitors used were synthesized as iodinated analogs; their binary complexes with renin may serve as single-site rational heavy atom derivatives. X-ray data beyond 2.8-A resolution have been collected by oscillation photography using the Cornell High Energy Synchrotron Source in Ithaca, NY.