The influence of the lattice of immune complexes on the C1q solid phase assay was examined using covalently cross-linked 125I-labelled immune complexes, separated into pools of varying and stable lattice. The C1q binding of antibodies alone, of Ag1Ab1, and of Ag2Ab2 could not be distinguished from each other statistically; but with increasing, higher lattices, immune complexes bound more efficiently to C1q. The binding of these immune complexes to C1q was also measured with 131I-labelled antibodies to IgG in the immune complexes. The detection of bound immune complexes by this indirect method showed the same order of binding efficiency as that observed by the direct measurement of immune complex binding. Up to a critical level, the binding of 131I-antibodies to IgG was proportional to the 125I-IgG in the bound complexes, and was independent of the lattice of complexes. This proportionality, however, was lost at higher levels of binding. The presence of serum diminished the binding of both large latticed and small latticed immune complexes, but serum did not alter the order of binding efficiency and the order of detection of binding using 131I-antibodies to IgG. The conclusion was reached that no single ideal standard for this assay can be currently designed to permit accurate quantitation of the concentration of immune complexes of varying lattice.