We conducted an investigation to clarify whether or not the levels of total, free, and functional protein S and C4-binding protein (C4bp) in plasma are decreased in systemic lupus erythematosus (SLE) patients, especially those with antiphospholipid antibody (aPL), which is known to be a causative factor of such complications as habitual abortion and arteriovenous thrombosis. Fifty patients with SLE were recruited as subjects of the study. Serum aPL (anticardiolipin, antiphosphatidyl serine, antiphosphatidyl inositol, and antiphosphatidic acid antibodies) were measured by ELISA. Lupus anticoagulant was determined by a PTT, KCT, and diluted RVVT. Furthermore, plasma concentrations of total, free, and functional protein S and C4bp were measured. There were no significant differences in the mean levels of total, free, or functional protein S and C4bp between aPL-positive, aPL-negative SLE patients, and the healthy controls. From these results, we concluded that the protein S level is not the sole factor causing complications, and that other factor(s) may be involved in the induction of such complications in this clinical setting.