The identification of antigens recognized by T cell responses has become fundamental for developing effective immunizations against viral infections. Lymphocyte proliferation and delayed-type hypersensitivity responses to porcine reproductive and respiratory syndrome virus (PRRSV) infection have been demonstrated. However, the polypeptide specificity of T cell responses to PRRSV is unknown. To identify the PRRSV polypeptides recognized by porcine lymphocytes two approaches were employed. First polypeptides of purified virions were separated by SDS-PAGE and particle suspensions obtained from nitrocellulose blots were used as antigens. Second, the polypeptides encoded by ORFs 2, 4, 5, 6, and 7 of the strain VR-2 332 were expressed as fusion proteins with a histidine tag in mammalian cells, using vaccinia virus as expression system. Significant antigen-specific proliferation responses to the matrix and envelope proteins from purified virions were obtained. This finding was supported by specific and dose-dependent proliferation responses to the recombinant polypeptides encoded by ORF2, 5 and 6 detected in virus-infected but not in control pigs. These results demonstrate that T-cell responses can be detected to individual PRRSV polypeptides. The greater response to the product of ORF6 than to the other PRRSV polypeptides indicates that the viral matrix polypeptide may have a major role in cellular immunity.