Immunoglobin (IgG) molecules with anti-2,4-dinitrophenyl activity were covalently cross-linked by using three cross-linking reagents. The resulting oligomers were separated into monomer, dimer, trimer, and heavy fractions. These stable assemblages of IgG molecules were capable of fixing dilute whole guinea pig complement in solution. When oligomers were further aggregated noncovalently into larger complexes, all were able to fix complement. Radioiodinated oligomers were attached to 2,4,6,-trinitrophenylsulfonic acid treated sheep red blood cells (N3ph-SRBC), and the number of bound molecules was determined from the cell-associated radioactivity. Complement-mediated lysis of N3ph-SRBC was then assayed over a range of levels of bound protein and at increasing concentrations of complement. The lytic efficiencies of all oligomers increased with the number of bound molecules, with complement concentration, with hapten density on N3ph-SRBC, and with oligomer size. The results suggest that two adjacent IgG molecules may not serve as a unit signal for triggering the complement cascade, but instead, initiation occurs with increasing efficiency as the size of cell-bound IgG clusters increases.