Abstract Avidin, a heat labile protein found in raw egg white, specifically binds biotin into a stable non-digestible complex preventing utilization of the vitamin by animals and microorganisms. It has been used often to induce biotin deficiencies and to bind biotin in isolated enzyme systems. Also, combinability with avidin has been employed to characterize analogs of biotin ( Burk and Winzler, 1943; Eisenberg, 1963; Birnbaum and Lichstein, 1965). Tausig and Wolf (1964) isolated a protein (streptavidin) from fermentation filtrates of streptomycetes which specifically binds biotin. The crystallized material was found to complex 1 μg of biotin per 150 μg of protein. Streptavidin differs from avidin in amino acid and carbohydrate content, UV adsorption, and electrophoretic mobility ( Chaiet and Wolf, 1964). The present report is concerned with a comparison of the combinability of avidin and streptavidin with various analogs of biotin.