Synthetic dye, aluminon has been demonstrated to cause swelling of isolated nuclei, decondensation of chromatin fibers, and dissociation of histones from chromatin. These phenomena are similar to those caused by high molecular weight polyanions such as heparin. In this paper, nuclear swelling with aluminon was studied in more detail in order to establish the role of aluminon as a probe for nuclei and chromatin. Studies were also done on the binding of aluminon to nuclei, and on its effect on RNA synthesis in vitro. Nuclear swelling was monitored from changes in turbidity of nuclear suspension. A stoichiometrical relationship was suggested between the amount of aluminon needed for maximal swelling and that of nuclear DNA in the system. The swelling velocity at pH 5.4 was much slower than that at pH 7.9 and the time course of the swelling was shown to be consisted of four distinct successive processes in which the swelling velocity was constant. Centrifugation of treated chromatin in a linear density gradient of sucrose revealed a decreased density of chromatin and dissociation of histones and certain nonhistone proteins from chromatin. Aggregation of arginine rich histones after dissociation was observed. The release of histones from chromation was also deduced from the fact that aluminon removed the restriction of chromatin template when transcribed with homologous RNA polymerases. Above results indicate that aluminon may be a useful tool for studies on structure and function of chromatin.