In this paper the concept of hapten is discussed , as well as the most used methods for conjugation of haptens (carboxylic acids, amines, nitro compounds, alcohols, phenols, carbohydrates, aldehydes and ketones) to carrier proteins. At present these hapten-carrier protein conjugates are useful as immunogens for obtaining polyclonal and monoclonal antisera employed in the production of immunodiagnostic reagents which represent one of the main Biotechnology applications. [ABSTRACT FROM AUTHOR]