The present invention relates to a streptococcal octapeptide AXYLXXLN, and preferably to the octapeptide AXYLZZLN, designated as a peptide associated with rheumatic fever (PARF) that, through its interaction with human collagen, plays a crucial role in the pathogenesis of rheumatic fever. PARF therefore represents a marker for rheumatic fever associated strains and provides a target for therapies, and in particular preventive therapies.