Cell-mediated immunity has been demonstrated to be a necessary component of immunity against viral infection. Methods to detect T-cell mediated immune responses to porcine reproductive and respiratory syndrome virus (PRRSV) infection were established both in vitro as lymphocyte proliferation and in vivo as delayed-type hypersensitivity response (DTH). Optimal conditions for detection of lymphocyte proliferation were determined by testing different antigen concentrations and various stimulation periods. The proliferation response to PRRSV was antigen-specific and dose-dependent. The kinetics of the T-cell proliferation response to PRRSV were analyzed after primary and secondary exposure to virus. Lymphocyte proliferation was first detected at four weeks post-infection (PI), peaked at 7 weeks PI, and declined after 11 weeks PI. The secondary response increased in magnitude. Experiments with blocking antibodies to porcine leukocyte antigens demonstrated that CD4+ T-cells were the major effector cells in the proliferation response. The in vivo response to PRRSV was shown by detection of a dose-dependent DTH reaction in infected pigs after intradermal challenge with UV-inactivated virus. These results demonstrate that pigs generate specific T-cell responses on PRRSV infection and provide a foundation for studying their role in protection.