The second step required in the application of avidin-biotin technology is to prepare an appropriate avidin-associated probe or probes for the desired application (for general reviews, see refs. 1 and 2). For example, a fluorescent form of avidin can be used for fluorescence microscopy, fluorescence-activated cell sorting, and in some cases, for immunoassay. Like-wise, an avidin-enzyme conjugate can be used for immunoblotting, immunoassay, light microscopy, and in some cases, electron microscopy. An immobilized form of avidin can be used for isolation purposes (see Note 1).