The bacteria Streptococcus agalactiae, part of normal human flora, produce an enzyme, hyaluronate lyase, which appears to contribute to the invasive capacity of this pathogen by degrading hyaluronan and chondroitin sulfates of the extracellular matrix of host tissues. The native enzyme, the product of the hylB(3502) allele, has a molecular mass of 111 kDa but undergoes an autocatalytic conversion to a smaller enzymatically active 92 kDa form. To initiate the determination of the catalytic mechanism of action of these enzymes, the 111 and 92 kDa enzymes were crystallized by a vapor-diffusion method using polyethylene glycol monomethyl ether 5000 and potassium thiocyanate as precipitating agents. The 111 kDa enzyme crystals are of poor quality and diffract X-rays to a very low resolution. However, the crystals of the truncated 92 kDa enzyme diffract X-rays to 2.1 A resolution. The crystal symmetry is C222(1) and the unit-cell parameters are a = 51.69, b = 157.03, c = 239.20 A (alpha = beta = gamma = 90 degrees ). The V(m) of 2.64 A(3) Da(-1) is consistent with the presence of one molecule of hyaluronate lyase in the asymmetric unit and a crystal solvent content of 53%. An isomorphous ethylmercuricthiosalicylic acid heavy-atom derivative diffraction data set has been obtained in order to solve the structure.