A new, sensitive and quantitative technique for the routine estimation of immune complexes has been devised. The assay involved the binding of immune complexes to Clq linked to the surface of plastic tubes; the amount of immune complex bound is then determined by adding radiolabelled anti-human IgG. Immune complexes were detected in twelve out of thirty-one patients with systemic lupus erythematosus. The test should prove useful in the diagnosis of immune complex diseases and will be valuable in the evaluation of disease activity.