Porcine reproductive and respiratory syndrome virus (PRRSV) non-structural proteins (nsps) play a key role in processing and maturation of the repertoire of structural and nsps of the virion, but little is known about the anti-nsp immune response. Here, it was hypothesized that pronounced antibody responses are generated to PRRSV nsp1 and nsp2, as they are present in infected cells and cytolytic infection releases viral proteins into interstitial spaces. Accordingly, nsp1 and nsp2 were cloned and expressed, and antibody responses in the sera of infected and vaccinated pigs were determined. Pigs mounted significant cross-reactive antibody responses that appeared equivalent to or greater than the response to nucleocapsid (N). Antibody reactivity to nsp1 and N was highly dependent on refolding of denatured proteins, suggesting that the porcine antibody response is directed primarily to conformational epitopes. The proteins reacted with sera from pigs infected with other PRRSV strains, indicating that multiple epitopes are conserved. Antibody responses to nsp1 and nsp2 were much higher than those to nsp4, which is encoded on the same RNA molecule and is equivalent in predicted antigenicity. These findings suggest either that nsp1 and nsp2 are highly immunogenic or that they are expressed at higher levels than nsp4 in PRRSV-infected cells, or both. Strong antibody responses to nsp1 and nsp2 may benefit the host by limiting potentially pathological consequences of viral protease activities encoded in these proteins that are released from dying cells. The identification of strain-specific antibody responses to a highly variable region of nsp2 may also provide the basis for immunoassays that differentiate serological responses of vaccines from field isolates.