Major poultry pathogens M. gallisepticum and M. synoviae share a gene encoding a putative cysteine protease CysP similar to papain cysteine protease (C1A subfamily). Comparison of the cysP gene sequences of 18 M. synoviae and 10 M. gallisepticum strains sequenced in this study showed polymorphisms, including deletions. Seven M. synoviae strains, including the type strain WVU 1853, had a 39 bp deletion in the 3' end of the cysP gene. In the same cysP region, all M. gallisepticum strains showed a deletion of 66 bp. Immunoblot analysis with specific antibodies demonstrated that M. synoviae strains expressed CysP, which was approximately 65 kDa. Both M. synoviae and M. gallisepticum were able to digest chicken IgG (cIgG). Incubation of cIgG (∼170 kDa) with M. synoviae or M. gallisepticum cells (∼15 h at 37 °C) resulted in a papain-like cleavage pattern of cIgG and fragments corresponding to the antigen-binding fragment of IgG (Fab, ∼45 kDa) and the crystallizable region fragment (Fc) of the IgG heavy chain (dimer of ∼60 kDa). Iodoacetamide (50 mM) prevented cleavage of cIgG by both Mycoplasma species. Following site-directed mutagenesis (eight TGA codons were changed to TGG) the cysP gene of M. synoviae ULB 925 was expressed as a His-tagged protein in a cell-free system. Purified recombinant CysP (rCysP; ∼67 kDa, pI∼8) cleaved cIgG into Fab and Fc fragments. This indicates that CysP is responsible for the cIgG cleavage caused by M. synoviae and, probably, by M. gallisepticum. This is the first evidence to our knowledge that mycoplasmas have enzymes that can cleave the host IgG and indicates a novel strategy used by M. gallisepticum and M. synoviae for prolonged survival despite the antibody response of their host.