Aft 3? essages in LTK cells, the TNL strain of metidcnbies vinis was further attenuaal either aitxnatively in LTK and ES K cell lines or straightly in ES K cell under the presence of BUdR of a concenttition of 1,000 mcgfml. xtivity and sblity of the )dnase of the mutantinvitro were stixiiai. Resuiis irdicated that the TK- mutant of TNL-LTK ESK could not replicate in HAT n-Mium. It was not vinilent day -old chicks and its viMence t’ rabbit was greatly raincal. A little reveithn of the vinilence was found when it was serially essaged 10 tines in RK- 13 cell without BUdR.
The TK- mutant was safe t pregnant sows. The control sow lcGt appetite and showed dp depression and fehile reflon for 4 days after challenge. On the other hand, the vaccinatJ sow had terapeiflie rise for one day only and did not show any chnicaisigns.