We have investigated the binding and activation of C1 by IgG-anti methotrexate antibody at cell surfaces. Under conditions where variation in cell surface hapten density had no effect on binding of IgG, the number of C1 (or its active form, C1) bound by the IgG was independent of hapten density. The ability of the C1 binding IgG complex to activate C1, however, was decreased with decreasing density of the hapten. The decreased ability to activate bound C1 was paralleled by decreased ability to activate the hemolytic sequence in whole complement. The results were interpreted to mean that binding of C1 was the result of aggregation (doublet formation) by IgG while activation of the bound C1 depended on changes induced in the IgG molecule by straddling hapten molecules at varying distances.