Using single-molecule force-clamp spectroscopy, where the distance between the AFM tip and the sample surface is fixed and a few parallel avidin-biotin complexes are kept stretched by a certain force, we were able to observe the formation of single avidin-biotin bonds. Perspectives to use such all approach to study association reactions at single-molecule level in the conditions resembling those characteristic for some processes in vivo (e.g. virus-cell membrane attachment) are briefly discussed. (C) 2008 Elsevier B.V. All rights reserved.