Recent clinical Streptococcus pyogenes isolates of the Ml serotype can be grouped according to the IgG-binding properties of their M proteins. One group expressed an IgG-binding M1 protein reactive with human IgG1, IgG2, IgG3, and IgG4 (type IIo); the other expressed a protein with predominant reactivity with human IgG3 alone (type IIb). Both IgG-binding protein phenotypes were equally resistant to phagocytosis in human blood; however, when they were injected into a skin air sac on outbred CD1 mice, all mice injected with Ml isolates of the type IIo phenotype were dead within 70 h, while only 40% of those injected with M1 isolates of the type IIb phenotype died within the same period. Bacteria recovered from the spleens of animals that died after injection with type IIb phenotype isolates demonstrated a change in their IgG-binding profile and were indistinguishable, in vitro or in vivo, from isolates displaying the type IIo phenotype.