Summary. Background: The antiphospholipid syndrome (APS) is characterized by the persistent presence of anti-β2-glycoprotein I (β2-GPI) autoantibodies. β2-GPI can exist in two conformations. In plasma it is a circular protein, whereas it adopts a fish-hook conformation after binding to phospholipids. Only the latter conformation is recognized by patient antibodies. β2-GPI has been shown to interact with Streptococcus pyogenes. Objective: To evaluate the potential of S. pyogenes-derived proteins to induce anti-β2-GPI autoantibodies. Methods and results: Four S. pyogenes surface proteins (M1 protein, protein H, streptococcal collagen-like protein A [SclA], and streptococcal collagen-like protein B [SclB]) were found to interact with β2-GPI. Only binding to protein H induces a conformational change in β2-GPI, thereby exposing a cryptic epitope for APS-related autoantibodies. Mice were injected with the four proteins. Only mice injected with protein H developed antibodies against the patient antibody-related epitope in domain I of β2-GPI. Patients with pharyngotonsillitis caused by S. pyogenes who developed anti-protein H antibodies also generated anti-β2-GPI antibodies. Conclusions: Our study has demonstrated that a bacterial protein can induce a conformational change in β2-GPI, resulting in the formation of antiβ2-GPI autoantibodies. This constitutes a novel mechanism for the formation of anti-β2-GPI autoantibodies.