The effect of poly(ADP-ribosyl)ation on native and H1-depleted chromatin was analyzed by gel electrophoresis, electron microscopy, and velocity sedimentation. In parallel, the interaction of automodified poly(ADP-ribose) polymerase with native and H1-depleted chromatin was analyzed. In H1-depleted chromatin histone H2B becomes the major poly(ADP-ribose) histone acceptor protein, whereas in native chromatin histone H1 was the major histone acceptor. Poly(ADP-ribosyl)ation of H1-depleted chromatin prevented the recondensation of polynucleosomes reconstituted with exogenous histone H1. This is probably due to the presence of modified poly(ADP-ribose) polymerase and hyper(ADP-ribosyl)ated histone H2B. Indeed, about 40% of the modified enzyme remained associated with H1-depleted chromatin, while less than 1% of the modified enzyme was bound to native chromatin. The influence of poly(ADP-ribosyl)ation on the chromatin conformation was also studied at the level of nucleosome in using monoclonal and polyclonal antibodies specific for individual histones and synthetic peptides of histones. In native chromatin incubated in the presence of Mg2+ there was a drop in the accessibility of histone epitopes to monoclonal and polyclonal antibodies whereas upon poly(ADP-ribosyl)ation their accessibility was found to remain even in the presence of Mg2+. In poly(ADP-ribosyl)ated H1-depleted chromatin an increased accessibility of some histone tails to antibodies was observed.