To further characterize the humoral immune response of pigs to porcine reproductive and respiratory syndrome virus (PRRSV), direct enzyme-linked immunosorbent assays (ELISA) were used to study the kinetics of antibody responses directed against PRRSV nonstructural proteins in pigs experimentally exposed to the virus. The highest immunoreactivities were against nsp1, nsp2, and nsp7. Using the recombinant nsp7 as an antigen, we validated a dual ELISA for the simultaneous detection and differentiation of serum antibodies against type I and type II PRRSV. Receiver operating characteristic analysis based on 1,334 known-positive and 1,357 known-negative samples showed good specificity (98.3% to type I and 99.3% to type II) and sensitivity (97.4% for type I and 99.8% for type II). To differentiate type I and type II PRRSV, 470 sera originating from experimentally inoculated pigs were tested, and positive sera were correctly differentiated in 469 of 470 samples. The capability of the nsp7 dual ELISA to detect serum antibody responses from pigs infected with various genetically different field strains was determined.The nsp7 dual ELISA possessed 97.6% agreement with the Idexx HerdChek PRRS 2XR ELISA. In further testing of Idexx ELISA suspected false-positive samples, the nsp7 dual ELISA resolved 98% of the samples as negative. Taken together, these results indicate that the nsp7 dual ELISA can be used as a differential test for PRRSV serology with high levels of sensitivity and specificity. This ELISA offers an additional tool for routine or follow-up diagnostics, as well as having substantial value in epidemiological surveys and outbreak investigations.