Abstract: The present invention provides an immunoassay of general applicability. The assay detects an antigen by its inhibition of the reaction between the combining site of two antibodies. One of the antibodies binds antigen, and the second antibody binds to the combining site or idiotype of the first antibody. It is preferable that both the antibody that binds the antigen and the anti-idiotypic antibody are monoclonal antibodies. The assay provides a method for measuring concentrations of single epitopes without requiring purified antigen, and it is particularly useful for measuring the quantity of any nonpurified antigen present in low concentration in a mixture of antigens. The use of monoclonal antibodies as reagents is advantageous in providing an essentially unlimited supply of highly specific and standardized reagents, without the batch-to-batch variation encountered when using conventional polyclonal antibodies.