Direct measurement of the intermolecular potential between a pair of charged actin filaments is essential for understanding their pathways of self-assembly. We have developed two methods that allow us to directly measure the adhesion energy between a pair of filaments held together by the depletion or any other attractive interaction. The sensitivity of our force measurement is in a picoNewton to femtoNewton range. Our data indicate that thermal fluctuations renormalize the adhesion strength in the weak binding limit. We are able to correlate changes in the adhesion strength with changes in the structure of actin bundle.