Abstract: A method and kit for determining the presence of a polyvalent ligand in an aqueous fluid. The method involves incubating the fluid with an immobilized antibody to the ligand to form an immobilized ligand-antibody complex, then incubating the immobilized ligand-antibody complex with a solution of a soluble complex of a second antibody (to the ligand) and a labeled binding protein, such as protein A. The labeled binding protein is specifically bound to the Fc portion of the second antibody. The unbound second antibody and labeled binding protein are washed from the immobilized complex, and the presence of bound labeled binding protein is determined as a measure of the concentration of the ligand in the aqueous fluid.