BVDV. Bovine viral diarrhoea/mucosal disease is one of the most important viral diseases of cattle. The natural prevalence is very high with approximately 80% of cattle being seropositive and 1-2% of these animals being persistently viremic animals due to immune tolerance which occurs after infection of the fetus (Kniazeff, 1973) . The infection rate has been increased by the uncontrolled use of live vaccines and by heterologous vaccines fortituously contaminated with BVDV virus (Kreeft et al., 1990) . Together with Hog Cholera and Border Disease virus of sheep, BVDV constitutes the pestivirus group. Bolin et al. (1991) have studied the frequency of contamination of fetal calf serum with BVDV and reported that 332 of 1608 raw fetal serum samples (20.6%) derived from the abbatoirs were positive for this virus, 224 of these samples (13.9%) contained antibodies against BVDV and 3.1% of the samples (50/1608) were positive for both, BVDV and antibodies against BVDV (Table XI ). They have also tested commercial fetal calf serum for cell culture and detec-ted BVDV in 47% of the samples (90/190): 88 contained non-cytolytic and only two contained cytolytic BVDV isolates. Two percent of these samples (3/190) were positive for Infectious Bovine Rhinothracheitis virus isolates. Wessman and Levings (1999) have reported similar results, indicating that 32 to 68% of fetal bovine serum samples (pooled one liter lots from two bovine fetuses) were rejected for presence of BVDV or antibodies against BVDV, in the period of 1990-1997.