Streptococcal inhibitor of complement (SIC) is a 31 kDa extracellular protein produced by a few highly virulent strains of Streptococcus pyogenes (in particular the M1 strain). It has been shown additionally to inhibit four further components of the mucosal innate response-lysozyme, secretory leucocyte proteinase inhibitor, human alpha-defensin 1 and the cathelicidin LL-37 which are all bactericidal against Group A Streptococci (GAS). We now show that SIC also inhibits variably the antibacterial action of hBD-1, -2 and -3. By enzyme-linked immunosorbent assay (ELISA), SIC binds strongly to hBD-2 and hBD-3, but not at all to hBD-1. Investigation of the antimicrobial action of beta-defensins hBD-1, -2 and -3 against GAS in two different buffer systems shows that both the killing efficiencies of all three defensins, and the binding of SIC to them, occurs more efficiently in 10 mm Tris buffer than in 10 mm phosphate. The lower ionic strength of the Tris buffer may underlie this effect. hBD-1 kills the M1 strain of GAS only in 10 mm Tris, but is able to kill an M6 (SIC negative) strain in 10 mm phosphate. The inhibition of hBD-3 by SIC is clearly of physiological relevance, that of hBD-2 is likely to be so, but the inhibition of hBD-1 occurs only at lower ionic strength than is likely to be encountered in vivo. Elastase digestion of SIC yields three major fragments of MW 3.843 kDa comprising residues 1-33 (fragment A); 10.369 kDa comprising residues 34-126 (fragment B); and MW 16.487 kDa, comprising residues 127-273 (fragment C). By ELISA, only fragment B binds to hBD-2 and hBD-3 and this may indicate the inhibitory portion of the SIC molecule.