Ehrlich ascites tumour cells were pulse-labelled with [3H]thymidine for 1 min or were treated with cycloheximide and labelled with [3H]thymidine for 45 min. The kinetics of digestion with micrococcal nuclease of both pulse-labelled and cycloheximide chromatins showed that they exhibited increased susceptibility towards the enzyme. At the same time their release from the nucleus was retarded and this was interpreted to mean that, unlike the bulk of chromatin, they were tightly bound to a fixed nuclear structure. When subjected to an equilibrium metrizamide-triethanolamine density gradient centrifugation both pulse-labelled and cycloheximide chromatins banded at higher density than control chromatin, which was an indication of their higher protein to DNA ratio. After a mild trypsinization, eliminating H1 and the nonhistone proteins, the pulse-labelled chromatin sedimented to the same density as control chromatin, and the cycloheximide chromatin sedimented to a density which was intermediate between those of control chromatin and free DNA. This result showed that the newly replicated chromatin had the same, and the cycloheximide chromatin half the amount of core histones present in control chromatin.