Our colloidal system is composed of antigens covalently coupled to polystyrene microspheres. When monoclonal antibodies are added to solution, they reversibly bind to the antigens causing aggregation. Unlike many other similar systems, our system is thermodynamicallyreversible: the aggregates will dissociate in the presence of free antigen, and the aggregation process reaches a state of thermodynamic equilibrium. This system is of interest as it can be used to model many different chemical, physical and biological processes. In particular, this system can be used to quantify the interactions between the biomoleculesattached to the microspheres1 and also to explore the statistical mechanics of reversible particle aggregation.