Radioactive iodine has been used to probe the relative reactivities of nucleosomal H4 tyrosine residues under various conditions of subphysiological ionic strength. We observe that tyrosine 72 of H4, which is not reactive over the range 20-150 mM NaCl, becomes the predominant site of iodination within H4 when nucleosomes are subjected to conditions of very low ionic strength. Conversely, the other H4 tyrosine residues, which are reactive within nucleosomes in solutions of moderate ionic strength (20-150 mM NaCl), become nonreactive when the ionic strength is reduced. This "flip-flop" in the H4 iodination pattern is the manifestation of a reversible nucleosomal conformational change. A method is presented which enables the conformational status of H4 in nucleosomes to be determined by simply electrophoresing the histones on a Triton gel after probing nucleosomes with labeled iodine. Using this technique, we demonstrate that the presence of H1 on one side of the nucleosome stabilizes a histone core domain on the other side so that all four tyrosines of H4 are maintained in their physiological ionic strength conformation even under conditions of no added salt.