Technical Abstract: Objective: To determine strain specificity and efficacy of the immune response of pigs following vaccination with selected strains of porcine reproductive and respiratory syndrome virus (PRRSV). Animals: 30 specific-pathogen-free pigs. Procedure: The study comprised 5 groups of 2-to 3-week-old pigs (6 pigs/group). On day 0, groups III, IV, and V were vaccinated with attenuated versions of PRRSV strains 8, 9, and 14, respectively. On day 21, groups II, III, IV, and V were exposed to a composite of the virulent versions of these same 3 strains. At necropsy (day 35) lungs and selected lymph nodes were examined for virus-induced changes. Blood (serum) samples obtained at weekly intervals and lung lavage fluids obtained at necropsy were tested for the presence of PRRSV and its strain identity. Results: The strain of PRRSV identified in the sera of pigs after vaccination, but before exposure to virulent PRRSV, was always the strain with which the pig had been vaccinated. Conversely, with 1 exception, the strain or strains of virus identified in sera and lung lavage fluids after exposure to virulent PRRSV were never the strain with which the pig had been vaccinated. There was no clear evidence that vaccination with any of the 3 attenuated strains provided immunity against the multi-strain exposure. Conclusions: Vaccine-induced immunity against PRRSV is at least partly strain related when pigs are exposed to virulent virus as early as 3 weeks after vaccination. Clinical relevance: Multi-strain vaccines may be necessary to provide timely and effective immunity.