Primary cells derived from explants or continuous cell lines immortalised/transformed by viruses can be contaminated by adventitious viruses. Primary cell cultures derived from animal tissues are seldom used for the production of vaccines for humans but are more frequently applied for veterinary use. When the donor animals are already latently infected with viruses the subsequent in vitro cultures of cells derived from these animals may be infected. If such primary cells are then used for the production of viral vaccines, these vaccines are likely to be contaminated with the adventitious virus. In the period between 1954 and 1961, when primary kidney cells from macaque or rhesus monkeys were used for the production of poliovirus vaccine, this vaccine was frequently contaminated by SV40. The source of this virus was from the kidney cells of infected monkeys (Sweet and Hilleman, 1960; Shah and Nathanson, 1976 ) (for details, see 'Passages via viruses infected animals'). Young immunocompetent rhesus or macaque monkeys can readily be infected with SV40 by the oral, intranasal, and subcutaneous routes, and viremia and viruria occur in these animals (Shah et al., 1969) . The use of such animals as source for kidney cells may lead automatically to a SV40 contaminated primary kidney cell culture because the virus may persist in the kidneys in a latent form (Shah et al., 1969) . Similar observations were made with secondary lamb kidney cells, which are widely used for the production of veterinary vaccines. In the case described, an attenuated Aujeczky's Disease viral vaccine was produced on lamb kidney cells. The master virus stock used for the production had become contaminated with the Border Disease virus due to the contamination of the cell culture used for its production. The vaccination of the sows with this vaccine during the first third of their pregnancy led to the infection of the fetuses which led to a disease similar to classical swine fever (Vannier et al., 1988) . Leiter et al. (1978) reported on the establishment of a mouse epithelial pancreatic cell line which was persistently infected by a polyoma virus. The origin of this infection was not completely clear, but it seemed probable that the mouse which was the source of these cells was also the source of the virus.