Porcine reproductive and respiratory syndrome (PRRS) is one of the major causes of economic losses to swine producers. The causative agent, the PRRS virus (PRRSV), has been tentatively classified as an arterivirus. PRRSV infect swine alveolar macrophages and induce persistent infection. In vitro, PRRSV replication is restricted to few continuous cell lines, none of which of porcine origin. The interaction of PRRSV with cells and the major structural proteins of the virus have been investigated in these studies, aiming to better understand the mechanisms governing cell tropism and replication in cells;Several cells were used to determine the mechanism of resistance to infection. PRRSV replication was restricted at several distinct steps: first, in most cases, resistance correlated with absence of viral receptors on target cells; second, Vero cells were found defective in virus uncoating; third, a mechanism following virus entry might inhibit virus replication because transfection of cells with viral RNA did not induce progeny virus in all cells tested;The mechanism of virus uptake into MARC-145 cells was also investigated. By analyzing the effect of lysosomotropic drugs on PRRSV infection of cells and electron microscopic analysis of cell-bound viruses, it was determined that PRRSV entered cells by a microfilament-dependent, receptor-mediated endocytic mechanism and that a low pH step was required during the initial stages of infection;The major structural proteins of PRRSV (E, M, and N) were cloned and expressed in a baculovirus system, in an attempt to identify the viral protein(s) that mediated binding to cell. None of the recombinant proteins bound to mammalian cells by using several binding assays. Nonetheless, these proteins were used to induce polyclonal sera that were used to detect the proteins in mammalian cells following virus infection. Using IFA staining of cells, proteins E and M were found circumscribing the cellular nucleus, whereas the N protein was located throughout the cellular cytoplasm;In conclusion, there are multiple mechanisms that might restrict PRRSV replication in cells. However, following binding to permissive cells, PRRSV enters through the endocytic pathway and, during the replication process, the major viral structural proteins accumulate at specific cellular compartments.