Reduction of the frequency of contaminations. Statistically, the use of fewer animals (kidneys) will increase the probability to establish virus free cultures. As primary kidney cells from monkeys can be amplified to reactor scale cultures by using microcarriers, the number of animals (kidneys) per batch could be considerably reduced (Van Wezel et al., 1978 , 1980 . The calculations done by Van Wezel et al. (1978) in- dicated very clearly that this approach could reduce the number of animals necessary for production purposes of Polio virus vaccine by 5-7 fold. Shah and Nathanson (1976) calculated the probability to obtain kidney cultures free from SV40 with respect to the number of animals used per production batch. By using one animal the frequency of SV40 contamination was about 20%. However, the frequency increased to 70% when the kidneys of two to three animals were pooled and was 100% when the kidneys of more than 10 animals were pooled, indicating very clearly that the increase of the number of animals per batch increased considerably the probability of the presence of virus contamination.