Abstract Over the past two decades, the avid in–biotin system has evolved to be an extremely useful tool in the fields of biology, biochemistry, and biotechnology. The extraordinary affinity (Kd ∼ 10−15 M) of avid in (or its bacterial relative, streptavidin) for the vitamin biotin forms the basis for using this system. This approach has found broad application, mainly for localization studies, medical diagnostics, and recombinant genetics. The principles and scope of avidin–biotin technology have been extensively described and catalogued, and the reader is referred to the series of reviews which are available in the literature (1–5) as well as the book which has recently appeared (6).