The screening of ligands against proteins plays important role in drug discovery and biological research. Using a dye labelled Streptavidin binding aptamer (SBA) as a competitive reporter probe, we found that adenosine bound to streptavidin specifically. Fluorescence spectral analysis showed that adenosine bound to both avidin and streptavidin with the K(ds) in the range of 0.1-0.2 mM, and these bindings can be blocked by biotin. Although streptavidin and avidin are well-known and widely used in bioanalysis, their biological role is still a riddle so far. Since adenosine is a ubiquitous physiological regulator present in cells, our finding provides new clues for the understanding of the functions of both proteins.