A three-stage chemical modification procedure [Lambert, S. F. & Thomas, J. O. (1986) Eur. J. Biochem. 160, 191-201; Thomas, J. O. & Wilson, C. M. (1986) EMBO J. 5, 3531-3537] for selectively radiolabelling lysine residues that interact with DNA has been used to investigate core histone--DNA interactions in sea urchin sperm chromatin, in particular to determine the binding site of the long N-terminal domain of sperm-specific H2B. Comparison of the patterns of radiolabelling of core histones from extended chromatin and nucleosome core particles (which lack linker DNA) reveals the regions of the histones involved in interactions with the linker. The results show that the N-terminal domain of H2B is bound to DNA outside the 146-bp nucleosome core, presumably to the linker DNA. H2A and H4 make no substantial contacts with the linker in extended chromatin; the N-terminal tail of H4 is bound within the core particle, but the N-terminal tail of H2A is not bound in core particles or in extended chromatin, and may therefore have a role in higher-order structure. H3, like H2B, makes contacts with DNA outside the 146-bp nucleosome core in its N-terminal region, as well as elsewhere, and probably interacts with the two 10-bp extensions that complete the two turns of DNA in the nucleosome and/or with the linker.