The catalytic domain of collagenase G from Clostridium histolyticum has been cloned, recombinantly expressed in Escherichia coli and purified using affinity and size-exclusion column-chromatographic methods. Crystals of the catalytic domain were obtained from 0.12 M sodium citrate and 23%(v/v) PEG 3350 at 293 K. The crystals diffracted to 2.75 A resolution using synchrotron radiation. The crystals belong to an orthorhombic space group, with unit-cell parameters a = 57, b = 109, c = 181 A. This unit cell is consistent with the presence of one molecule per asymmetric unit and a solvent content of approximately 53%.