In the experiment pigs of two distinctly different populations were challenged with PRRS virus. The lines differed in physiological responses to virus (body temperature, weight gain, ability to replicate virus, and lung lesions), indicating that genetic variation exists. This suggests that breeders could select directly for resistance to PRRSV. However, such selection is very difficult to implement, would be very expensive, and is certainly not practical. It would require continuous challenge of all pigs to PRRSV and then selection of those with the optimum response as measured by the physiological traits. A much more practical procedure would be to genotype animals for genes conferring resistance and select directly for these genes. This kind of selection can be practiced without infecting pigs with PRRSV. Such selection cannot be practiced today because the genes have not been identified. The Phenotypic data and the tissues collected in this experiment will be used to search for these genes with the aim of developing practical selection methods.