The in vitro production of viral vaccines began with the demonstration that explanted embryonic tissue could be used for the production of poliovirus (Enders et al., 1949) . Subsequently primary cells, in particular primary monkey kidney cells, were used for the production of this virus. Although the use of such cells was very convenient and the first accepted way to produce a viral vaccine, the source of these cells (the primary monkey kidney cells) was associated with a number of problems particularly with the introduction of a number of viral contaminats. The advantage of the use of primary cells for the vaccine production system was that it was the first in vitro system for the production of viral vaccines, that high titers of viruses could be obtained, and that the system could be scaled up to a reactor stage by using microcarriers . However, it has been recognised that, this system could be prone to contamination originating from the donor animals. The frequency of contaminants may have been due initially because, the animals were caught in the wild with no control of the disease risks with these animals, leading to a high incidence of contaminations by adventitious viruses. Stones (1977) reported that 40-80% of the cultures of kidney cells from Vervet monkeys were positive for adventitious agents. As mentioned, many of the viruses which infect primates are pathogenic for humans, the species barrier is being able to be crossed (Eloit, 1997) .