3CL protease, a viral chymotrypsin-like proteolytic enzyme, plays a pivotal role in the transcription and replication machinery of many RNA viruses, including porcine reproductive and respiratory syndrome virus (PRRSV). In this study, the full-length 3CL protease from PRRSV was cloned and overexpressed in Escherichia coli. Crystallization experiments yielded crystals that diffracted to 2.1 A resolution and belong to space group C2, with unit-cell parameters a = 112.31, b = 48.34, c = 42.88 A, beta = 109.83 degrees . The Matthews coefficient and the solvent content were calculated to be 2.49 A(3) Da(-1) and 50.61%, respectively, for one molecule in the asymmetric unit.