IgG is the preferred substrate. In addition, literature reports have shown that Endo S does not cleave human serum IgM or IgA *
* Collin, M., Olsen, A. EndoS, a novel secreted protein from Streptococcus pyogenes with endoglycosidase activity on human IgG. The EMBO Journal (2001) Vol.20 No.12 pp.3046-3055.