In a study of the complement system in patients with Behcet's disease, hemolytic activity of CH50 and ACH50 and the complement components C4, C3, C5, C8, C9 and factor B were assessed and compared to the CRP, α2-globulin level and erythrocyte sedimentation rate which are parameters of inflammation. As the controls, those suspected of having Behcet's disease and those with diabetes mellitus and rheumatoid arthritis were compared with those having Behcet's disease. (1) In Behcet's disease, both CH50 and ACH50 were high. Of the complement components, not only C4 and C3 but also C5, C8, C9 and factor B showed an increase. Particularly, the increase of C9 and factor B were conspicious. (2) The complement components showed an acute phase protein charactor which was stronger in C5, C8 and C9 than in C4 and C3. It appeared that the strongest was C9 among the complement components studied. (3) The definite type of Behcet's disease showed higher values of CH50, C5 and C9 than the probable type of Behcet's disease. This fact will be helpful in differentiating one from the other.