Radioimmune assays were developed to assay the binding of complement components C1q, C1s and C4 to antibody aggregates and to cell-bound antibody. The binding of the components was compared with the haemolytic activity and with the capacity to form the C3 convertase activity in the presence of excess C2. The destruction of whole complement and of C4 activity is similar per 1,000 molecules of antibody in aggregates and cell-bound antibody, as is the binding of C1g and C1s, the latter being in a 1:2 molar ratio. The binding of C4 is about 12 times greater, per 1,000 molecules of antibody, on cells than in aggregates. However, the effective C4 molecules, as judged by the formation of C3 convertase activity, are much more similar on cells and aggregates. An assembly mechanism of the early components of complement on antibody-coated cells, which is compatible with these results, is suggested.

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