Unique roles have been identified for the histone octamer in the formation and stabilization of higher order chromatin structures. Histone octamers were assembled onto 12 tandem repeats of Lytechinus 5 S rDNA, at either saturating or subsaturating ratios. The extent of oligonucleosome folding and intermolecular association in divalent salts was monitored using analytical and differential sedimentation techniques. Saturated oligonucleosomes (12 nucleosomes/DNA) sedimented at 29 S in very low salt buffer. In 1.0-2.0 mM MgCl2, saturated oligonucleosomes formed a maximally folded 55 S structure whose extent of compaction was equivalent to that of classical higher order 30-nm diameter chromatin structures. These results are in marked contrast to those obtained previously in NaCl, where the maximally folded oligonucleosome species sedimented at only approximately 40 S (Hansen, J. C., Ausio, J., Stanik, V. H., and van Holde, K. E. (1989) Biochemistry 28, 9129-9136). Mg(2+)-dependent formation of the 55 S conformation was inhibited by histone octamer depletion; the maximum sedimentation coefficient observed for rDNA molecules containing 10-11 nucleosomes in 2.0 mM MgCl2 was only 40 S. Above 2.0 mM MgCl2, the equilibrium was progressively shifted toward formation of large associated oligonucleosome species. The implications of these results to the mechanism of chromatin folding and its relationship to the biological activity of the chromatin fiber are discussed.