The ability of IgG antibodies with different hapten specificities to fix C1 and activate C as a function of hapten density on a red cell surface was investigated. Rabbit anti-methotrexate and anti-folinic acid IgG antibodies in a mixture were highly efficient in fixing C1 and activating C when cells carried simultaneously high levels of both haptens. We wished to find out whether in a C-activating IgG complex both IgG molecules had to be in a form that could activate C1. By reducing hapten density of one of the haptens on a double labeled cell, complexes were generated where only one in a pair of IgG molecules was in the activating form; such a pair had the same activating efficiency as a pair in which both IgG molecules were in the activating form. It was concluded that cooperative activation of C in C1-binding IgG complexes required only one IgG in the complex to be in the activating form.