In experimental immunization of sheep and goats against anthrax the vaccine containing our own protective antigen (PA) whose serological activity was evaluated by the titre 1:16 was used. PA was adsorbed on aluminium hydroxide gel at the ratio 30 parts of gel and 100 parts of antigen. In the control experiment the doses of the virulent anthrax strain were as follows: 10(6) (sheep) and 5.10(6) (goats). The single vaccination provided only 60% protection, the repeated 100% protection. By the titration of antibodies of the vaccinated animals was proved that the peak titres (1:80 to 1:160) were reached two months after vaccination. Then the titre decreased gradually, and in approximately two months' time it returned to the initial value. As apparent from these experimental results, a reliable protection by PA vaccine can be achieved only by double immunization, best at 14-day intervals.