Antibodies are studied in terms of their properties as molecular recognition elements using an antigen monitoring biosensor as a means of examining sensitivity and selectivity. This approach requires only a small amount of antibody for study and is illustrated for a model system in which two monoclonal antibodies and one polyclonal antibody to 2,4-dinitrophenol are compared on the basis of their association constants and cross-reactivity patterns with heterologous haptens.