The viral proteins of porcine reproductive and respiratory syndrome virus (PRRSV) were expressed in a baculovirus expression system. They were evaluated for protection of pigs against the respiratory disease and for function in virus-cell interactions. The open reading frames 2 to 6 products were confirmed to be membrane associated proteins as they were detected on the surface of viable insect cells by an immunofluorescence assay. Immunization of pigs with vaccines comprised of a combination of GP2, GP3 and GP4 or a combination of GP5, M and N proteins induced partial protection, but immunization with combination of GP5 and M proteins failed to induce protection. Immunization of pigs with the first two vaccines significantly reduced the severity of clinical respiratory disease (P < 0.001), severity of pneumonia (P < 0.05) and duration of viremia compared with those of control group. These results suggest that in addition to envelope proteins, N protein may also play an important role in inducing host protection. For further characterization of these proteins, monoclonal antibodies (MAbs) were prepared. Most of the MAbs were found to be against conformationally dependent epitopes. Antigenic variation was found in PRRSV field isolates and thirty-three PRRSV isolates were divided into three groups based on their reactivity with the MAbs to GP2, GP3 and M proteins. Early virus-cell interactions in PRRSV infection was evaluated with a virus binding assay on CRL11171 cells. The recombinant ORFs 2 to 6 products failed to block PRRSV binding, implying that there were necessary interactions between viral structural proteins. PRRSV receptor was shown to contain protein moieties as protease treatment of cell monolayers reduced virus binding. The receptor activity was reduced from the cell surface with octylglucoside treatment, but no specific protein band was found by immunoprecipitation of the cell surface extracts, suggesting that the PRRSV receptor is present in low concentration. An 18-kDa viral protein was found to bind to the cells by an adsorption assay. Glycosylation did not affect the binding ability of this protein. The protein was immunoprecipitated with MAb against PRRSV M protein, suggesting its potential involvement in virus binding and/or entry.