This study investigated the process of PCV2-induced apoptosis and the effect of PCV2 inoculation on calcium homeostasis in piglet lymphocytes in vitro.PCV2-inoculated lymphocytes exhibited chromatin condensation, chromatin segregation, the appearance of membrane-enclosed apoptotic bodies, and DNA fragmentation.Moreover, the proportion of apoptotic cells increased significantly in PCV2-inoculated lymphocytes compared with controls.These results demonstrate that PCV2 induces lymphocyte apoptosis.Some evidence suggests that an alteration in the intracellular free Ca2+ concentration ([Ca2+]i) could cause apoptosis.We measured elevated [Ca2+]i in PCV2-inoculated lymphocytes for 12 h or 24 h compared with controls.Our results support that PCV2-induced apoptosis may be relative to [Ca2+]i.In addition,calmodulin (CaM) was increased in PCV2-inoculated lymphocytes for 12 h compared with controls.The amount of CaM-dependent protein kinase Ⅱ (CaMKⅡ) did not change with PCV2 inoculation.We infer that the increased [Ca2+]i can bind CaM protein, but functions independently of CaMKⅡ.Inositol 1,4,5-trisphosphate receptor (IP3R)-1 mRNA expression increased with PCV2 inoculation, whereas plasma Ca2+-ATP4 mRNA expression decreased.A decreased Ca2+-ATP4 level may inhibit Ca2+ efflux, and the increased IP3R-1 may trigger Ca2+ release from the endoplasmic reticulum.Both of these changes may contribute to increased [Ca2+]i.