Colostrum is involved in early protection against Mycoplasma hyosynoviae in piglets. Four week old pigs raised in infected farms are protected against M. hyosynoviae. Specific antibodies may have some protective effect against development of M. hyosynoviae arthritis. Immunologically naive 4-week-old pigs are susceptible to M. hyosynoviae infection. M. hyosynoviae has the potential of inducing an antigen-specific T-cell response.