Streptococcus equissp.zooepidemicus (S.zooepidemicus, SEZ) is one of the common pathogens that can cause septicemia,meningitis, and mammitis in domesticated species.Identify the virulence factors of this bacteria is an effective way of clarify its pathogenic mechanism.We employed the In vivo-Induced Antigen Technology (IVIAT) to find bacterial genes only express or up-regulated when pathogen infect into host (IVI genes).Convalescent-phase sera from pigs infected with SEZ were pooled, adsorbed against invitro antigens, and used to screen SEZ genomic expression libraries.Upon analysis of the identifiedproteins, we were able to assign a putative function to 36 of the 43 proteins.These includedproteins implicated invirulence 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 of pigs.