The initial identification of nucleosomes has been broadened to the present demonstration of heterogeneity among nucleosomes both in size and in the proteins associated with them. We intend to continue our investigation of histone variant composition of chromatin in four different cell types of Friend leukemia and their comparison with normal erythroid differentiating cells. We shall now focus on analyzing chromatin differences in the active as compared to inactive portions of the different cell types. We shall use specific endonucleases and specific genetic probes. The availability of a cell which can be induced to differentiate in vitro allows us the ability to examine the change in chromatin structure, including the nucleosomal composition of active genes upon induction with chemical inducers.