Abstract: Serum CH₅₀ and C₃ in 70 cases of psoriasis were determined, and compared with normal control. The C₃ value of psoriasis group was significanfly lower than normal subjects(P<0.001), CH₅₀ showed no significant differeace(P>0.05). No difference existed between progressive stage and static stage.Decreased C₃ value indicated that there are antigen antibody immunocomplex, which can bind the complement, exists in the intercellolars pace of stratum corneum, leading to appearance of psoriasis lesions.