The development of cell-mediated immunity has been known extremely important in clearing porcinereproductive and respiratory syndrome virus (PRRSV) in infected pigs. However, the PRRS immunol-ogy regarding the interaction of T-cells and PRRSV proteins is poorly understood. To identify the T-cellimmunodominant epitopes on the membrane (M) protein of PRRSV, a series of 31 overlapping pen-tade capeptides covering the entire M protein were designed and synthesized. These peptides werescreened by ELlspot analysis for their capabilities to elicit interferon-gamma (IFN-y) responses inthe peripheral blood mononuclear cells (PBMCs), which were collected from pigs immunized with attenuated PRRSV HuN4-F112 Strain and Challenged Wlth highly pathogenic HuN4 strain. After threerounds of screening, 4 peptides (M3, MG, M8 and M12) were shown to elicit high expression of IFN-y. The stimulation Of high IFN-y transcription In PBMCs by these 4 peptides was further Confirmed inreal-time PCR. The Sequence alignment revealed that the epitope represented by peptide M6 was fullyconserved in all of examined 42 North American genotype II PRRSV isolates and the epitopes representedby peptides M3, M8 and M12 showed 2-4 amino acid replacements. The finding of 4 T-cell immunodom-inant epitopes in the M protein of PRRSV will be beneficial to the understanding of the development of cell-mediatedimmunity.