Streptococcal protease precursor, secreted by the human pathogen Streptococcus pyogenes, becomes activated to a cysteine protease. The precursor and the mature enzyme appear to contribute to S. pyogenes virulence. The precursor protein was crystallized in the form of very thin flexible flakes. X-ray diffraction data were collected to 3.15 A resolution at 100 K using synchrotron radiation. The crystals are monoclinic, space group P2(1), with unit-cell parameters a = 41.6, b = 136.0, c = 156.7 A, beta = 95.7 degrees, and contain four copies of the protein in the asymmetric unit.