Abstract: During the detection on safety and effectiveness of porcine reproductive and respiratory syndrome （PRRS） vaccine, pigs which antigen and antibodies of PRRS negative were needed. In this experiment, 135 pig serum samples which came from five counties （districts） of Guanzhong Area, Shaanxi Province were tested by ELISA and RT-PCR method. The results showed that antibody-positive rate and antigen positive rate of PRRS were 6.67% （9/135） and ll.ll% （14/126） respectively. It proved that the presentments and control of PRRS in scatter-feed herds were insufficient, there is great danger of PRRSV infection.