Abstract: Methods and compositions are provided for identifying a group A Streptococcus in any of a variety of physiological samples, such as blood, saliva, throat swabs, cerebrospinal fluid, brocheolar lavage material, and biopsy material. The positions used include an extracellular cysteine protease or a fragment thereof, obtainable from S. pyogenes and containing at least one conserved epitope, or nucleic acid encoding the cysteine protease or fragment thereof. The compositions also find use in eliciting a protective immune response against a group A streptococcus infection.