Two isolates of NSDV/GV were available to us for these studies, one a highly tissue-culture passaged isolate of the virus from Uganda (and therefore notionally NSDV), the other an isolate from India (and therefore notionally GV) which had been passaged a limited number of times in mouse brain or BHK 21 cells. Although these two isolates have been shown phylogenetically to be the same virus [2] , we will refer to them as GV and NSDV in this paper to indicate the two isolates. The GV isolate proved to be still pathogenic in sheep, while the NSDV isolate was nearly completely attenuated, and the GV isolate is therefore probably nearer to wild type virus.