Abstract: This paper reports the study on the activation of alternate pathway of complement system during the pathological course of systemic lupus erythematosus (SLE). Serum levels of complement ACH₅₀, B factor CH₅₀, C₃, C₄ were measured in 36 patients with SLE.The results revealed that the alternate pathway of complement system in addition to the classical pathway were activated. B factor was not only related to the pathogensis of SLE but with the activity of the disease. The change of B factor correlated with the changes of C₃ and C₄, especially in the active phase of SLE.