Immunogenicity of vaccine against enzootic abortion of ewes (EPO) was evaluated in sheep and laboratory white mice. The vaccine contained purified and formalin-inactivated corpuscules of Chlamydia psittaci. Experiment was performed on ten sheep, Slovak Merino breed, which were negative before vaccination in serological assays and blastic transformation of lymphocytes (BTL) tests. The animals were immunized subcutaneously with 2 ml of vaccine which contained 100 micrograms (sheep No. 1-4) and 20 micrograms (sheep No. 5-8) corpuscules of C. psittaci. Control group (No. 9-10) received the same volume of physiological saline. The second dose of vaccine was given one month after the first dose (No. 1-2--100 micrograms each, and No. 5-6--20 micrograms each). Blood for serological evaluation and BTL test was taken before vaccination and 1, 3, and 6 months after vaccination. The sheep which were given the second dose of vaccine were also evaluated two weeks thereafter. Antibody response in complement fixation reaction (CFR) and enzyme-linked immunosorbent assay (ELISA) was compared with the results of BTL. While the antibody response evaluated by CFR was sporadically positive after administration of the higher dose of vaccine only, in ELISA all sera were positive except one lower dose of vaccine (Tab. I). In all post-vaccination intervals positivity was confirmed by BTL test irrespective of the size and number of vaccine dose (Tab. II). Immunization of mice with one dose (100 micrograms) of vaccine significantly but not completely reduced multiplication of C. psittaci in the lungs of mice (Tab. III).