Micrococcal-nuclease digestion of rat liver nuclei selectively released mononucleosomes associated with ADP-ribosylated [Caplan, Ord & Stocken (1978) Biochem. J.174, 475-483] histone H1. Two classes of mononucleosome were detected, those that leaked out during digestion and those that were subsequently released by 5mm-sodium phosphate buffer (pH6.8)/0.2mm-NaEDTA. The former, from which histone H1 had been dissociated, contained 140-base-pair-length DNA and core histones;the latter contained core particles and mononucleosomes with histone H1 and 200-base-pair-length DNA. When normal liver nuclei were phosphorylated with [gamma-(32)P]ATP, dissociated histone H1, which could be separated from core particles with Sephadex G-200, showed (32)P uptake. (32)P uptake into histones H2A and poly(ADP-ribosyl)ated H3 was appreciable in core particles, but was less evident in nucleosomes still containing histone H1. When [(3)H]-thymidine was given to partially hepatectomized rats in S-phase, 5-10min pulses in animals of over 300g body wt. showed the presence of high-specific-radioactivity DNA in released core particles and mononucleosomes compared with DNA retained in the nuclear pellets. Mononucleosomes from rat livers in S-phase with new, [(3)H]lysine-containing histones, had higher (32)P incorporation in histones H1 and their core histones, than for di- or tri-nucleosomes. Thermal-denaturation properties of control and phosphorylated mononucleosomes and core particles were very similar; removal of histone H1 and non-histone chromosomal proteins in 0.5m-NaCl markedly increased the proportion of DNA ;melting' below 70 degrees C.