The purpose of this study was to evaluate the effect of a subsequent infection of porcine reproductive and respiratory syndrome(PRRS) virus to pigs with A pleuropneumonia. Twenty three 7-week-old commercial pigs were infected intratracheally with PRRS virus and/or A pleuropneumoniae serotype 5. Serum antibody titers were examined by an enzyme-linked immunosorbent assay(ELISA) and proportion of porcine leukocyte subpopulations in peripheral blood was examined by flow cytometry. In this experiment, antibodies against PRRS virus and A pleuropneumoniae were detected at 2 weeks and 1 week postinfection and the number of antibody positive pigs were gradually increased. And in proportion to leukocyte subpopulations in peripheral blood of pigs infected with A pleuropneumoniae compared with pigs administrated with saline, the proportion of PoCD4 and N cells were increased(P<0.1). Furthermore, in proportion to leukocyte subpopulations in peripheral blood of pigs infected with PRRS virus followed by A pleuropneumoniae compared with pigs administrated with saline, the proportion of MHC class II, PoCD4 and B cells were significantly increased(P<0.1). The results indicated that dual infection with PRRS virus and A pleuropneumoniae induced the stronger immune responses associated with macrophages and Th cells in pigs than single infection with PRRS virus or A pleuropneumoniae.