A significant increase in the survival of the BHV 1 variants (8/-2 and III/2D) was found during successive cycles of seroneutralization. The second cycle of 40-minute neutralization was survived by 39-39.8 per cent of the viral particles, while only 1.17 per cent of the first cycle. In case of the III/2D variant 21.4 per cent of the particles survived the third cycle of 40-minute neutralization. The diameter of plaques of the 8/-2 variant was 1.13 mm and was significantly larger compared with the parental strain Agille (p<0.01). On the other hand the diameter of plaques of the III/2D variant was 0.67 mm and was significantly smaller than the diameter of plaques of the III/2 (p<0.001) strain.