A longitudinal study was conducted to characterize the humoral immune response of pigs to porcine circovirus type 2 (PCV2), the postulated causative agent for postweaning multisystemic wasting syndrome (PMWS). Eight-week-old cesarean-derived, colostrumdeprived (CD/CD) pigs were inoculated with a purified isolate of PCV2 and kept for 35 days post-inoculation (PI). Serum samples were collected from all pigs on day 0 and, thereafter, every 7 days until termination of the study. Naïve young pigs were shown to be susceptible to PCV2. PCV2-specific antibodies were detected by an indirect fluorescent antibody test at day 7 PI and after, whereas neutralizing antibodies were not detected until day 28 PI. Western immunoblot analysis of the sera demonstrated three virus-specific proteins with molecular mass of 28, 28.5 and 35 kD. By comparing the appearance of antibody with protein specificity of antibody response, the 28-kD protein was highly immunogenic and specific for PCV2, suggesting that the 28-kD protein may provide the antigenic basis for the development of diagnostic tests for the detection of PCV2-specific antibody. Other two proteins may be associated with virus neutralization.