If adapting influenza virus from one propagation host to another propagation host, it is important to passage the virus at least three times in the new host to allow for adaptation. Each passage should be tested for virus growth, and it may also be important to check the titer of each passage to ensure that the virus is not being lost during adaptation to the new host. When changing the propagation host, it is also important to consider changes that may need to be made to the viral growth medium, temperature and cell confluency (if appropriate) based on the properties of the new host. For propagating influenza virus in tissue culture, the viral growth medium should not contain Fetal Bovine Serum (FBS), as serum will neutralize the activity of TPCK-treated trypsin.