A very important consideration is the capacity of the irradiated serum to support cell growth. By performing long-term standard cultures (three passages in a medium supplemented with 5% of the irradiated sera), Plavsic et al. (1999) were able to show that in principle all tested cell lines could be cultivated, but also that different cells reacted relatively differently on the radiation doses used. Whereas low passage BHK cells, Vero (only slightly), and CHO cells displayed an inverse relationship between growth and radiation dose, high passage BHK cells and the human diploid fibroblasts, WI-38 and MRC-5 -the latter are of special interest for vaccine production and virology -did not display growth decline as a function of radiation dose (Table XV ). None of the tested cell lines showed a modifed morphology.