Infection of BHK cells with foot-and-mouth disease virus (FMDV) causes a thorough change in the electrophoretic profile of whole nuclear histones. It consists in the disappearance of histone H3 and the appearance of a new polypeptide (Pi) which migrates between histones H2A and H4 on SDS-polyacrylamide gels. Protein Pi is detected at 2 hr postinfection (pi), the time in which viral RNA synthesis begins to increase, and reaches equimolecular amounts with the remaining core histones 1 hr later, when the disappearance of histone H3 is almost complete. Labeling of cells prior to infection demonstrates that Pi is not a novo product but the result of a viral-induced processing of a host precursor synthetized beforehand. Protein Pi comigrates with histone H2A/B in acetic acid/urea polyacrylamide gels and it shares common major peptides with histone H3 under controlled proteolysis with protease V8 or trypsin. The mononucleosomal and nucleosomal DNA pattern analysis after micrococcal nuclease treatment of nuclei from infected and mock-infected cells did not show any significant differences even though after 3 hr (p.i.), protein Pi replaces histone H3 in the nucleosomal structure. It was concluded that FMDV infection is responsible for a specific modification in the nucleus of infected cells which leads, after 3 hr (p.i.), to a complete histone H3 protein Pi transition in the nucleosomes.