The free Beta-hCG ELISA test is based on the principle of a solid phase enzyme-linked immunosorbent assay.1 The assay system utilizes a unique monoclonal antibody directed against a distinct antigenic determinant on the free Beta-hCG. Mouse monoclonal anti-free-Beta-hCG antibody is used for solid phase immobilization (on the microtiter wells). A goat anti whole hCG antibody is in the antibody-enzyme (horseradish peroxidase) conjugate solution. The test sample is allowed to react sequentially with the two antibodies, resulting in the free Beta-hCG molecules being sandwiched between the solid phase and enzyme-linked antibodies. After two separate 30 minute incubations at 37