The 125I-Clq binding test was modified in order to allow for the detection of immune complexes in native unheated human serum. Indeed, heat-inactivation (56 degrees, 30 min) was found to reduce the Clq-binding activity of immune complexes mixed with native serum. This effect was not observed when EDTA was added to the native serum before mixing the immune complexes. The modified 125I-Clq binding test was performed in two steps: first, the tested native serum sample was incubated for 30 min at 37 degrees C with 0.13 M EDTA in order to prevent the integration of 125I-Clq into the intrinsic Clqrs complex, second, 125I-Clq and polyethylene glycol (final concentration 2.5%) were added to this mixture, and further incubated for 1 hr at 4 degrees C. Under these conditions, free Clq remained soluble whereas Clq bound to macromolecular complexes was precipitated. The competitive effect of intrinsic Clq and the interference of other substances such as DNA or bacterial LPS were very limited. The modified Clq binding test was applied to the clinical investigation of 44 patients with systemic lupus erythematosus; and increased Clq binding activity (Clq-BA) was observed in 91% of the samples. The level of Clq-BA was found to be significantly correlated to the DNA-binding capacity and to the decrease of the level of some complement components.