Antibodies can bind with high affinity and specificity to molecules of virtually any shape and to antigens ranging from small organic compounds to large proteins. These characteristics have led to the widespread use of antibodies as laboratory reagents, in diagnostic tests, and for therapeutic purposes. The immune system produces antibodies by a process of natural selection. In this article we describe how the strategy of natural selection can be used in bacteria to produce antibodies with novel characteristics and binding properties. This technology is opening up new diagnostic and therapeutic applications of antibodies.Antibody Production in VivoIn vivo, antigen-driven . . .