A C1q binding assay is presented which is suitable for use in comparison of the binding ability of different antibodies, and which allows the quantitative determination of their binding constants. The assay system uses IgG bound to a hapten-derivatized Affigel support. No non-specific binding is observed to a DNP-derivatized support, allowing the use of anti-DNP antibodies. With mouse anti-DNP hybridoma IgGs it was found that C1q binding followed the series IgG2a greater than IgG2b much greater than IgG1, in accordance with the complement fixing ability of these subclasses. Since it is relatively simple to couple any antigen to Affigel , this assay system should be generally applicable to any antibody-antigen system.