We demonstrate that the arginine-rich histones H3 and H4 can introduce torsional constraints on closed circular DNA with a concomitant compaction of the nucleic acid. SV40 DNA I complexed with H3 and H4 appears relaxed in electron micrographs and contains particles of 75 +/- 10 A in diameter along the DNA. SV40 DNA I is contracted 2.75 +/- 0.25 fold by all the four smaller histones and 2.6 +/- 0.4 fold by H3 and H4 alone. The arginine-rich histones can cause the topological equivalent of unwinding the DNA close to one Watson-Crick turn per particle formed. Spherical nucleoprotein complexes morphologically similar to isolated nu bodies or nucleosomes are obtained by association of H3 and H4 with 140 base pair length DNA isolated from chromatin core particles. These reconstituted particles sediment at 9.8S, as compared to 10.8S for native core particles, and contain a tetramer of the arginine-rich histones. None of these specific alterations in DNA structure is seen om complexing the slightly lysine rich-histones H2A and H2B to DNA. Our data provide further evidence indicating that the arginine-rich histones are the major determinants of the architecture of DNA within the chromatin core particle.