Abstract
During passage of the influenza A strain CAM in eggs in the presence of anti-CAM serum, three true-breeding variants were isolated. The variants Ic, Is and SP7 showed increasing resistance to inhibition by anti-CAM serum. The evidence indicated that virus population changed in the course of passage as a result of the appearance of successive variants capable of preferential survival in the experimental environment. The strains Is and SP7, unlike the parent strain CAM, were pathogenic for mice inoculated intranasally. The implications of these findings are discussed.