This experiment serves to underline the following concepts: (a) the inherent specificity of antibody-antigen binding, (b) the bivalent nature of antibody binding, and (c) the value of antibodies as diagnostic reagents. The experiment is simple and relatively inexpensive to perform, and serves to underpin the theory of antibody structure and function as taught in many basic biochemistry and immunology courses. (C) 1998 IUBMB. Published by Elsevier Science Ltd. All rights reserved.