The goal of this work was to test the role of the histone tails in the emergence of attractive interactions between nucleosomes above a critical salt concentration that corresponds to the complete tail extension outside the nucleosome [Mangenot, S., et al (2002) Biophys. J. 82, 345-356; Mangenot, S., et al (2002) Eur. Phys. J. E 7, 221-231]. Small angle X-ray scattering experiments were performed in parallel with intact and trypsin tail-deleted nucleosomes with 146 +/- 3 bp DNA. We varied the monovalent salt concentration from 10 to 300 monovalent salt concentration and followed the evolution of (i) the second virial coefficient that characterizes the interactions between particles and (ii) the conformation of the particle. The attractive interactions do not emerge in the absence of the tails, which validates the proposed hypothesis.