In two experiments 22 sheep have been immunized with BLV attenuated by UV. The vaccinated animals were' challenged with an active BLV. A dose of vaccine was higher in the first experiment. A seroconversion has appeared in all the vaccinated animals within 3—4 weeks. A challenge caused a considerable increase of the titre of specific antibodies. Both in challenged and in unchallenged sheep the antibodies persisted till the end of the experiment, i.e. for more than 1200 days. In the vaccinated but not challenged animals BLV was not found in blood. It has been stated that vaccination protected animals from an active BLV only when a dose of the virus was low. Large doses of the virus destroy immunological resistance inducing a nodular form of enzootic bovine leukosis.