In general, viral contaminations of cell lines cannot be treated and contaminated cultures should be discarded, with the exception of LDV. This virus causes a life long viremia in infected mice without any clinical signs, and each sample of these animals is virus contaminated. Because LDV requires primary mouse macrophages for replication it cannot survive repeated in vitro subcultivations, leading to a loss of this virus in infected in vitro cultures. Another elimination method is the passage of the contaminated cell line/tumor in another species, for example nude rats (Nicklas et al., 1993; Nakai et al., 2000) .