Mouse hybridomas are of particular concern because, first, these cells have been created by fusion of mouse spleenocytes with mouse myeloma cells, second, many hybridoma cells have been cultivated in animals, and third they are used for the produc-tion of injectables. This signifies that mouse viruses are potential contaminants of these cell lines and their products. These viral contaminants can be divided into two groups; group 1 contains viruses which are also known to cause human diseases or to be able to infect human cells, while group 2 contains other mouse viruses (Table V) (Minor, 1996) . Although Ectromelia virus is listed in group 2, cultures infected with this virus are only processed in the special P4 unit available at the NIH (Hay, 1991) . Ectromelia, a member of the orthopoxviurs group, is a natural pathogen in mice, and is able to replicate in all mouse lymphoma lines, in some hybridoma cell lines, and in BS-C-1 cells (Buller et al., 1987) . Consequently, the ATCC has screened its collection of murine cell lines but no characteristic cytopathic effects have been observed (Hay, 1991) . Moore (1992) listed the mouse viruses which had been detected in production cell banks of hybridomas: LMCV, MVM, Sendai, LDH, and epizootic diarrhea virus of infant mice. The present view is that hybridoma cell lines should be tested for the viruses indicated in Table V , and only those should be used for biotechnological applications which are free of these viruses. The only acceptable viral particles in bulk supernatants from hybridoma cell lines are those of endogenous origin.