Abstract: In order to investigate how blood compositions are changed by contagious diseases,7 hybrid pigs were infected by inoculating each with 1.25 billion live Streptococcus suis Subcutaneously.Severe hematological changes could be observed at 5 and 10 days after inoculation,which was characterized by hemorrhagic anemia ,hypoproteinenvia and hyperleukocytosis,and accompanied by high erythrocyte sedimentation rate and incresed munber of blood platelets.Among lymphocytes,T-and living cells decreased,and B-and dead cells increased signifificantly,although there was no marked change in the proportion of lymphocytes to neutrophilic cells in the blood of infected animals.Intravenous injections of sulfadimidne +trimethoprim to infected animals continously for 6 days (twice a day) from the 9th day after infection ,were able to improve the hematological and immunological picture significantly and make the animals recover.The interactions among animals,bacteria and therapeutic agents are discussed.