Virtually all strains of the human pathogenic bacterium Streptococcus pyogenes express a highly conserved extracellular cysteine protease. The protein is made as an inactive zymogen of 40,000 Da and undergoes autocatalytic truncation to result in a 28,000-Da active protease. Numerous independent lines of investigation suggest that this enzyme participates in one or more phases of host-parasite interaction, such as inflammation and soft tissue invasion. Replacement of the single cysteine residue (C-192) with serine (C192S mutation) resulted in loss of detectable proteolytic activity against bovine casein, human fibronectin, and the low-molecular-weight synthetic substrate 7-amino-4-trifluoromethyl coumarin. The C192S mutant molecule does not undergo autocatalytic processing of zymogen to mature form. Taken together, these data support the hypothesis that C-192 participates in active-site formation and enzyme catalysis.