Crystalline Klebsiella aerogenes urease was found to have less than 0.05% of the activity observed for the soluble enzyme under standard assay conditions. Li2SO4, present in the crystal storage buffer at 2 M concentration, was shown to inhibit soluble urease by a mixed inhibition mechanism (Ki's of 0.38 +/- 0.05 M for the free enzyme and 0.13 +/- 0.02 M for the enzyme-urea complex). However, the activity of crystals was less than 0.5% of the expected value, suggesting that salt inhibition does not account for the near absence of crystalline activity. Dissolution of crystals resulted in approximately 43% recovery of the soluble enzyme activity, demonstrating that protein denaturation during crystal growth does not cause the dramatic diminishment in the catalytic rate. Finally, crushed crystals exhibited only a three-fold increase in activity over that of intact crystals, indicating that the rate of substrate diffusion into the crystals does not significantly limit the enzyme activity. We conclude that urease is effectively inactive in this crystal form, possibly due to conformational restrictions associated with a lid covering the active site, and propose that the small amounts of activity observed arise from limited enzyme activity at the crystal surfaces or trace levels of enzyme dissolution into the crystal storage buffer.