A protein from Streptococcus pyogenes, serotype M1 has been characterized. This protein, called protein SIC, plays a role in S. pyogenes pathogenicity and virulence. It inhibits hemolysis by interacting with the plasma proteins clusterin, and members of the cystatin protein super family such as histidine rich glycoprotein (HRG). The protein comprises at least one of the following partial amino acid sequences: (a) glu thr tyr thr ser arg asn phe; (b) asp trp ser gly asp asp trp pro glu asp asp trp; (c) arg ser gly val gly leu ser gln tyr gly trp ser; (d) trp ser ser asp lys lys asp glu thr glu asp lyr thr; (e) gly thr gly tyr glu lys arg asp asp trp gly gly pro gly; and (f) lys arg asp asp trp arg gly pro gly his ile pro lys pro. The protein and antibodies specific for the protein can be used in analytical procedures for determining the presence of virulent Streptococcus pyogenes in a sample. The protein can also be used in vaccine compositions.