Method of determining the presence and/or concentration of a polyvalent antigenic substance in a fluid. The method comprises the steps of incubating the fluid with labeled antibodies to the substance to form a first labeled immunochemical complex and then incubating that complex with immobilized antibodies to the substance to form a second labeled complex which is separated from the incubation medium. The amount of label in the second complex provides a means for detecting and/or quantitating the substance in the fluid. By reversing the sequence of incubation steps in a "two-site" or sandwich assay, greater sensitivity is achieved and an intermediate washing step is eliminated.