The intraperitoneal, the muscular, and the nasal infection with a virus, containing material (blood of a cow, affected by leukosis, with 48100 leucocytes in a cubic mm and 92% of lymphocytosis) and cell suspension FLK, producing leukosis virus, provokes leukosis infection. It was established that the peroral and the intraconjunctival application of virus material does not provoke leukosis till the 6th month (term of observation). With saliva and semen, obtained from positive serologic reagents with persistent lymphocytosis also does not provoke infection. By the immunofluorescent test, in the infected sheep were observed antibodies against the summary glycoprotein antigen, which appear after 1-2 months with continuous seroconversion till 18th month and precipitation titer 1:2 till 1:64. Among 30% of the sheep, after the 5th and 8th month, was established leucocytosis, up to 18800 leucocytes, and persistent lymphocytosis--91-93%.