VC2002, isolated from postweaning multisystemic wasting syndrome (PMWS)-affected pig, is a mixture of two porcine circovirus genotype 2b (PCV2b) viruses, K2 and K39. Preliminary experiments disclosed short-term adverse effects of K39, but not K2, on porcine foetuses. These findings led to the hypothesis that infection of immuno-incompetent foetuses with K2 confers a status of immunotolerance, and postnatal super-infection with K39 triggers PMWS. To explore this hypothesis, nine 55-day-old foetuses were inoculated in utero (three with K2-104.3TCID50, three with K39-104.3TCID50 and three with medium), and foeto-pathogenicity examined. At 21 days post-inoculation (dpi), K2 did not induce pathology, whereas pathological effects of K39 were evident. Twenty-four 45-day-old foetuses were subsequently inoculated to examine the long-term effect of K2, including six with K2-high dose-104.3TCID50, six with K2-low dose-102.3TCID50 and 12 mock-inoculated controls. Both doses resulted in ifve mummiifed foetuses and one live-born piglet each (69dpi). K2 was recovered from all mummies. K2 and K2-speciifc antibodies were not detected in serum of the two live-born piglets at birth, indicating full control of K2 infection. The K2-low dose-infected piglet was immunostimulated at day 2, but not the K2-high dose-infected piglet. Both non-stimulated and stimulated K2-infected piglets were super-inoculated with K39 at day 6 or 8 (taken as 0 days post super-inoculation). Low viral replication was observed in the non-stimulated K2-K39 piglet (up to 103.3 TCID50/g;identiifed as K39). In contrast, viral replication was extremely high in the stimulated K2-K39 piglet (up to 105.6TCID50/g) and identiifed as K2, indicating that K2 infection is controlled during foetal life, but emerges after birth upon immunostimulation. However, none of the piglets showed any signs of PMWS.