Immunoassays were first developed over 30 yr ago. The radioimmunoassay for insulin described by Yalow and Berson (1) heralded a new era in the use of antibody reagents for the quantification of proteins and peptides. The Nobel Prize-winning research revolutionized analysis by virtue of much improved specificity and sensitivity coupled with ease of application to large numbers of samples. Subsequently, a number of further innovations have increased the power and utility of immunoassay. Of particular importance was the methodology for generating monoclonal antibodies described by Kohler and Milstein (2), theoretically allowing the production of unlimited amounts of identical antibodies (as opposed to the mixed populations making up polyclonal preparations). Second, the use of enzyme-linked immunosorbent assays (ELISAs) has obviated the need for handling radioisotopes (3) and consequently widened the utilization of immunoassays from clinical research to virtually all areas of biological analysis (4). More recently, the production of recombinant antibody fragments (5), using a variety of expression systems, offers the next leap forward. Such technical innovations will offer a faster and more reliable means to synthesize reagents of the desired affinity and specificity, taking immunotechnology on into the next century (6).