The GAS M49 591 chromosomal background was generated previously by insertional inactivation of the ska gene (16). The construction of the epf, emm49, and prtF2 mutant strains from GAS M49 591 has been described previously (17?0). The GAS strains were cultured in Todd-Hewitt broth (THB; Invitrogen) at 37 under a 5 CO2 and 20 O2 atmosphere. Materials. Pooled normal plasma and plasma deficient in