Streptococcus equi ssp. zooepidemicus (S. zooepidemicus, SEZ) is one ofthe common pathogens that can cause septicemia, meningitis, and mammitis in domesticated species. Identify the virulence factors ofthis bacteria is an effective way of clarify its pathogenic mechanism. We employed the In vivo-lnducedAntigen Technology (MAT) to frnd bacterial genes only express or up-regulated when pathogen infect into host (M genes). Convalescent-phase sera from pigs infected with SEZ were pooled, adsorbed against in vitro antigens, and used to screen SEZ genomic expression libraries. Upon analysis of the identified proteins, we were able to assigy a putative function t0 36 0f the 43 proteins. These included proteins implicated in virulence and adaptation; intermediary; gene replication, transcription and expression; energy metabolism; transport and those with unknown functions. We marched a step on the road of exploring the key genes which help SEZ survive in the host. This study also provided some new molecular targets data of studying on streptococcal disease ofpigs.