Porcine reproductive and respiratory syndrome virus (PRRSV) is a recent and widespread pathogen in the U.S. swine population. PRRSV infects cells of the macrophage/ monocyte/dendritic lineages which are important antigen presenting cells (APCs) of the immune system. Using flow cytometric (FACs) analysis, we demonstrated that PRRSV infection decreases the expression of the major histocompatibility complex (MHC) glycoproteins on the cell surface of infected macrophages. This decrease in MHC protein expression may reduce the ability of the macrophages to present viral antigens to the appropriate lymphocytes. The potential lack of viral antigen presentation may play a crucial role in the persistent viremia observed in PRRSV-infected pigs.