As for the other bovine derived viruses, BPyV is an ubiquitous virus and about 40% of the calves are seropositive in the first month after birth. In the subsequent months this seropositivity decreases to about 11% at an age of one year, however, the older the animals become seropositive again and the final percentage of seropositivity is beyond 80%. It was also shown that bovine fetuses were infected in utero, leading to the presence of antibodies against this virus in fetal bovine serum batches (in about 12% of the tested batches). Despite this rather high incidence of infection in fetuses, no known disease is associated with this virus, neither for cattle nor for humans. Using PCR, 14/20 European serum batches (70%) contained BPyV DNA sequences . A similar frequency of contamination was observed in North American serum batches (Kappler et al., 1996; Van der Noordaa et al., 1999) . There was no correlation between the PCR results and the presence/absence of antibodies against BPyV, however, could show that all PCR-positive sera contained infectious BPyV. IBR or BHV-1. BPyV can infect calf kidney cells, and monkey kidney cells (Vero, BSC-1, CV-1, RITA) but also human embryonic kidney cell cultures (Waldeck and Sauer, 1977; Wognum et al., 1984) . The virus does not seem to replicate in mouse 3T3 cells, nor in the human embryonic lung cell WI-38 (Waldeck and Sauer, 1977) . The BPy virus is known to lead to cell transformation and tumorigenesis, which is induced by the expression of the large-T antigen .