Objective Serum pharmacological assessment:the effect of Rubia yunnanensis extract on the immune function stimulated by superantigens of streptococcus. Methods Using cell culture technique, mouse spleen lymphocytes and peritoneal macrophages were culture with superantigens of streptococcus and serum of Rubia yunnanensis. Lymphocytes proliferation stimulated by superantigens of streptococcus measured by MTT assay. NO level in mouse peritoneal macrophages affected by Rubia yunnanensis at different concentrations were determined and compared. Results Serum of Rubia yunnanensis significantly inhibit the proliferation of lymphocytes stimulated by superantigens of streptococcus, compared with normal control. NO content of macrophages culture in Rubia yunnanensis extract group were lower than those of control. In these tests, the effect of the extract by alcohol is more obvious than that of extract by water. Conclusions Rubia yunnanensis extract has effect in inhibiting immune function and anti-infect activity, it could be applied in clinical practice for immunoregulation and treatment of psoriasis.