Abstract: To explore the interaction between porcine reproductive and respiratory syndrome virus (PRRSV) and porcine circovirus type 2 (PCV2) in pathogenesis， the virological kinetics and immunological responses in piglets to PRRSV and PCV2 co-infection were analyzed. Twenty healthy， six-week-old piglets were randomly divided into control group， PRRSV or PCV2 infected group and PRRSV/PCV2 co-infected group with 5 piglets each. After infection， serum and different tissue samples， including heart， liver， lung， kidney， pancreas， spleen， thymus， tonsil， inguinal lymph node and mesenteric lymph node， were used for PRRSV and PCV2 quantitation by real-time PCR， and serum was used for PRRSV- and PCV2-specific antibody detection by indirect ELISA and indirect IFA， respectively. The proliferation activities of peripheral blood lymphocytes (PBLC) were detected by 3-[4，5dimethylthiazol]-2，5-diphenyltetrazolium bromide (MTT). The results showed that PRRSV- and PCV2-loads in both serum and different tissue samples from co-infected group were significantly higher than those of PRRSV or PCV2 singly infected group. Antibodies to PRRSV and PCV2 developed later and reached significantly lower levels in serum from PRRSV/PCV2 group than from PRRSV or PCV2 group. The proliferation activities of PBLC in three virusinfected groups were suppressed and the degree of severity was shown as follows: PRRSV/PCV2 group > PRRSV group > PCV2 group. These results indicate that PRRSV and PCV2 co-infection can promote the replication of both PRRSV and PCV2 in vivo and synergistically suppress the immune responses of the co-infected piglets.