(b) γ Irradition (Plavsic et al., 1999) . γ irradition (using a 60 Co source) is a very efficient and straightforward means for inactivating many different virus types. As animal derived substances such as serum can be contaminated by adventitious viruses, γ irradition is, after routine quality control for virus detection, the best method to increase the safety of using serum in the production of animal cell culture derived biologicals. Using PETG (polyethylene terephthalate G copolymer) bottles with 500 ml of frozen serum (-40 • C) inoculated with model virus (Hanson and Foster, 1997) , validation experiments have been performed to determine the optimal radiation dose to inactivate relevant bovine and porcine viruses, and in parallel to assure that the irradiated serum has still a sufficient growth supporting ability. Plavsic et al. (1999) could show that at a radiation dose of 25 kGy, all tested viruses (Bovine Reovirus, Porcine Parvovirus, Canine Adenovirus, IBR, and BVDV) showed a significant decline in titer. An exposure of 35 kGy led to titers of all viruses tested falling below the detection level (≤0.5 TCID 50 ml −1 ). Even very resistant viruses, such as the Porcine Parvovirus, could be reduced to below the detection level. For all viruses tested the log reduction factor was at least 6.78 (Table XIV) . Willkommen et al. (1999) reported an overview on virus inactivation and removal from serum and serum substitutes. With respect to the efficiency of PPV inactivation by γ irradition they indicated that even after application of a radiation dose of 40 kGy, a TCID 50 of 5.3 per ml was observed, indicating that the log reduction was only about 2. This difference with data published by Plavsic et al. (1999) might be due to differences in the design of the respective studies. However, with respect to the other viruses tested (BVDV, IBR, PI-3, reovirus 3), no differences in the inactivation doses were observed. It should be mentioned here, that sera are normally irradiated using a dose of 20 to 25 kGy. For veterinary use, the radiation dose has to be 35 kGy.