The action of acidomycin and its analogs on avidin, as the experimental model of the biotin-protein complex, during the formation of avidin-biotin complex (A.B-complex) was examined in vitro. It was found that these compounds failed to free biotin estimated by L.arabinosus method against inactivation by avidin. It was concluded that A.B-complex cannot be employed as a model of experiment designed to obtain some answers for the increasing urinary biotin excretion in acidomycin-injected rabbits.