Objective: To study the red cell immune function and natural killer (NK) cell activity in the development of endometriosis. Methods: The red cell immune functions of 44 patients with endometriosis were assayed by means of erythrocyte C3b receptor rosette test and erythrocyte Ic rosette test. The NK cell activities in peripheral blood was assayed by means of MTT assay and compared with that of controls. Results: The RC3bR and NK cell activity of patients with endometriosis were lower than those of controls (P<0.05), and decreased as the stage of endometriosis increased. The RIcR of patients with stageⅢ,Ⅳendometriosis was higher than that of control (P<0.05). Positive linear relativity was found between RC3bR and NK cell activity in patients with endometriosis (r=0.665 9, P<0.01). Conclusion: The defects of red cell immune function and NK cell function may be related to the pathogenesis of the endometriosis.