Detection of C1q in serum, in serum-C1 and within immune complexes, inhibits hemolytic activity of C1 by binding to C1q in the liquid phase (antibody concentration necessary to inhibit 50 % of hemolytic activity in the fluid phase is 0.85 μg/mL). With C1 bound to immune complexes a dose dependent increase of C1 activity was observed at antibody concentrations of 1.5 μg/mL. Useful for detection of C1q-bearing immune complexes in patients with rheumatic disorders.