The stability of 2 H. bovis antigens prepared by different methods was examined by the ELISA. It was found that the activity of H. bovis secretory antigen kept at - 18°C and -70° C and full antigen stored at - 18°C was significantly decreased as early as 6 months after storage. Lyophilized, secretory H. bovis antigen showed constant, high activity and may be used for one year at least.