Day-old SPF leghorn chicks were divided into four groups and raised separately in isolators.Group 1 chicks were inoculated s.c,with 0.2 mL M D vaeeine and 0.5 mL thymus extract in .the neck,group 2 chicks were inoculated with 0.2 mL MD vaccine(immunized control)only,group 3 and 4 chicks were not injected with any vaccine or other substanee.Group l to 3 birds were challenged with the virulent BJMDV-1 strain on day 13. Death occurred on day 17 and 24 post-infection for groups 3 and 2 birds respectively.For the groups that reeeived thymus extract,death occurred on 28 days P.I.,No death occurred in group 4.Group 1 showed 40% protection rate against MDV whereas the protection rate in group 2 was only 15.0% In addition,the group that reeeived immunengancement, had higher T lymphocyte level in the peripheral blood than group 2.Results of this experiment indicated that the thymus extract had the potential for improving immunizaton against MD.