There have been very few developments that markedly affect the need to greatly revise the text from the last version of this book. This is testament to the fact that hetero- neous enzyme-linked immunosorbent assays (ELISA) provide ideal systems for dealing with a wide range of studies in many biological areas. The main reason for this success is test flexibility, whereby reactants can be used in different combinations, either attached passively to a solid phase support or in the liquid phase. The exploitation of the ELISA has been increased through continued development of specifically produced reagents, for example, monoclonal and polyclonal antibodies and peptide antigens coupled with the improvement and expansion of commercial products such as enzyme-linked conjugates, substrates and chromogens, plastics technology and design of microwell plates, inst- mentation advances and robotics. However, the principles of the ELISA remain the same. There has been some rearrangement of chapters plus addition of three new ones dealing with charting methods for assessing the indirect ELISA, ruggedness and robustness of tests-aspects of kit use and validation, and internal quality control and external quality management of data, respectively. These reflect the need to control what you are doing with ELISA and to exploit the method to its full extent. I do not apologize for dealing with the same areas in different ways a number of times, as it is imperative that principles are understood to allow planning, operation, and control of ELISA.
