Water-soluble carbodiimide crosslinks histones 1A and 1B to histone 2A and to semi-histone A24 in chromatin from mouse cells. The identities of the histone species present in the crosslinked dimers were determined by fingerprinting. The molar ratio of H1--A24 to H2A is the same as the molar ratio of A24 to H2A in these cells. The H1-H2A crosslinks form equally well in whole nuclei, lysed nuclei, and H1-containing mononucleosomes isolated from a sucrose gradient. These results suggest that there exist major H1 interactions within the nucleosome.