Eukaryotic chromatin consists of DNA complexed with histones and other proteins. This association is essential for some of the most important cellular processes, such as DNA replication and transcription, therefore the mechanisms involved in chromatin remodelling are of basic interest. The elemental structural unit of chromatin is the nucleosome, a bead-like structure composed of a short length of DNA wrapped around a core histone octamer [1]. The repeating nucleosome cores further assemble into a second level of organization, the 30 nm chromatin fibre, which is stabilized by histone H1 via its association with the internucleosomal linker DNA [2]. Prothymosin α (ProTα) is an abundant small acidic nuclear protein widely distributed in mammalian cells and tissues [3]. The expression of the ProTα gene is related to cell proliferation [4–8] and is activated by c-myc genes [9,10]. Nevertheless, in spite of its abundance and wide distribution, until now the biological function of ProTα has remained obscure, although accumulating experimental evidence points to a role for this protein in chromatin organization. In support of this view are its localization and abundance inside the cell nucleus [4,11–13] as well as its highly acidic character (pI 3.5). In this regard, anionic regions have been identified in several chromatin and chromosomal proteins [14]. Moreover, ProTα shares significant homology with nucleoplasmin, a chromatin-remodelling protein that is very abundant in the nucleus of Xenopus [15] and is capable of interacting with histones in itro [16,17]. The present work was an attempt to determine whether or not ProTα has a role in chromatin structure. For this purpose, we used as a biological system HL-60 cells stably transfected with plasmids expressing the ProTα cDNA in either the sense or the antisense orientation. With this system, we have previously shown that overexpression of ProTα stimulates cell proliferation, shortening the G " -phase of the cell cycle, which interferes with differentiation [4].