Experiments were carried out to use the agar gel diffusion test (AGDT) in the study of bovine leukosis. An antigen was obtained from FLK-24 cells with a latent leukosis virus infection, which proved specific and suitable for AGDT. A total of 134 serum samples were examined applyig AGDT, for the presence of specific antibodies, originating from cattle of herds in which leukosis had been hematologically demonstrated. Investigated were also 56 sera of animals presenting an unknown clinical picture. Positive reaction was established in 70 per cent of the sera taken from herds with hematologically demonstrated leukosis. The specificity of AGDT was confirmed through establishing specific antibodies also in sera of lambs experimentally infected with a virus from a FBL-J 5 cell line with a latent infection. It is admitted that AGDT is a specific, sensitive, and readily applicable method, which can successfully be used in the diagnostics of leukosis in this country.