Sites of interaction between histones H3 and h4 have been probed by investigating complex formation, firstly between histone H4 and three peptides cleaved by chemical means from histone H3 (residues 1-90 and 1-120 using cyanogen bromide and residues 42-135 using N-bromosuccinimide), secondly between histone H3 and two peptides cleaved from histone H4 (residues 1 - 84 using cyanogen bromide and residues 38-102 using chymotrypsin) and thirdly between the H4 peptide (residues 38-102) and the three H3 peptides (residues 1-90, 1-120 and 42-135). The criterion for complex formation is the appearance of characteristic perturbed resonances in the aromatic region of the 270 - MHZ proton resonance spectrum of the peptide mixture. It is concluded that loss of 37 N-terminal residues from histone H4 and 41 N-terminal residues from histone H3 does not prevent complex formation, whilst the loss of 18 C-terminal residues from H4 and 45 C-terminal residues from H3 does prevent it; that last 15 C-terminal residues of H3 are, however, not required for forming a complex. The regions important for complex formation are therefore defined as residues 42-120 in histone H3 and residues 38-102 in histone H4.