By using electron microscopy, the retroviruses present in cell culture supernatant have been as high as 10 9 particles per ml (Moore, 1992) . About one in 10 4 -10 6 particles is infectious. In this context, Froud et al. (1997) presented data from Lonza Biologics (former Celltech Biologics), indicating that all hybridoma cell lines processed by the company produced retroviral particles. Five to six percent of the cells produced infectious mouse ecotropic retrovirus, whereas almost all (about 85%) mouse cell lines tested produced low levels of infectious mouse xenotropic retrovirus (X-MLV) when the cell banks were tested.