Abstract: A flavin adenine dinucleotide (FAD) derivative of the formula: ##STR1## wherein n is an integer from 2 through 10, and preferably is 6. The derivatives are intermediates in the preparation of FAD-labeled conjugates useful as reagents in specific binding assays for determining ligands (e.g., antigens and haptens) in liquid media such as serum. Also provided are certain FAD-labeled conjugates, particularly an FAD-theophylline conjugate.