C(1)q, the first component of C(1), was isolated from normal human sera according to Thunild's method. Specific antibody to C(1)q was produced in rabbits. Fluorescein isothiocyanate (FITC) was labelled on the antibody. Renal tissues from 35 patients with systemic lupus erythematosus (SLE) were stained with FITC labelled antibody to C(1)q as well as C(3), IgG, IgM and IgA. The following results were obtained. 1. Glomerular deposition of C(1)q was frequently seen in patients who had massive deposition of immunoglobulins and C(3) along the glomerular basement membrane. The distribution of C(1)q is similar to that of immunoglobulins and C(3). 2. Positive C(1)q deposition on glomeruli was associated with low levels of serum C(1)q and the presence of complement fixing antibody to native DNA. 3. Serial renal biopsy of 15 patients with SLE showed that a marked decrease of C(1)q deposition was observed after steroid therapy. These results indicate that glomerular deposition of C(1)q is of importance to evaluate the clinical and immunopathological activity of lupus nephritis.