Through a skin-allergic reaction were subjected to a study cattle of different ages (calves-sucklings, growing calves and grown up calves), originating from 4 different from the epizootic point of view cow-farms--one latent focus, two foci with an apparent infection and one cow-farm in good health conditions. On the healthy cow-farm the skin-allergic reaction and the virus neutralizing reaction were not marked. On the farms affected with latent infection and with manifestation of abortions with cows, the skin-allergic reaction with calves till the age of 3 months was negative, whereas the virus neutralizing reaction was usually positive; with the calves at the age of about 6 months the skin-allergic reaction began slightly to become positive. The virus neutralizing reaction was also positive. With the grown up animals both reactions were positive. In the development of recently brought clinical infection among the calves, without affecting their mothers, both reactions were negative with newly born calves, whereas from the second month onward, they began to become positive.