The effect of interferon alpha (lFN-alpha) to inhibite the replication of PRRS virus on MARC-145 cells was studied. The rerults indicate that replication of PRRS virus was reduced on MARC-145 cells that treated with IFN-alpha at the concentration of 1250 IU and 2500 IU/ml and completely inhibited at the concentration of 5000 IU/ml. There is difference significantly in virus titration between the cells treated with and without treating with IFN-alpha was determined, with the virus titers of 10 exponent 30 - 10 exponent 45 TCID50/ml, 10 exponent 25 - 10 exponent 40 TCID50/ml and less than 10 exponent 10 TCID50/ml from MARC-145 cells infected with PRRS virus after treating with IFN-alpha at the concentration of 1250 IU, 2500 IU and 5000 IU respectively, compared to the titer of 10 exponent 5.5 - 10 exponent 60 TCID50/ml of control cell group. In addition, the effect of inhibiting the replication of PRRS virus by IFN-alpha on MARC-145 was also depended on the time of treating IFN-alpha: before, at the same time and after inoculating cells with PRRS virus. Injection of the killed PRRS vaccine in combination with IFN-alpha was stimulated antibody response in pigs and antibodies were able to be detected by ELISA at day 21 after vaccination, whereas pigs injected with either only killed PRRS vaccine or only IFN-alpha and control group were negative ELISA. This study showed that IFN-alpha inhibites the replication of PRRS virus strains on MARC-145 cells and using IFN-alpha in combination with killed PRRS vaccine is able to stimulate antibody response in pigs.