We analyze the nucleosome core assembly reaction which is mediated in vitro by a protein previously purified from Xenopus laevis eggs, now named nucleoplasmin in reference to its occurrence in the soluble phase of the nucleus of a wide range of vertebrate cell types. Nucleoplasmin is present in solution as a pentamer. We use nuclease digestion analysis to show that the protein assembles bona fide nucleosome cores in vitro from purified histones and DNA. Nucleoplasmin itself binds neither to DNA nor to the nucleoprotein particles which it assembles in vitro. However, it interacts with histones in vitro in such a way that histones no longer adhere to negatively charged surfaces. We have found no evidence for sterically specific interactions with particular histones. The initial rate of the nucleosome core assembly reaction mediated by purified nucleoplasmin in vitro is essentially identical with the rate of the nucleosome assembly reaction which occurs in the cell-free extracts of Xenopus eggs from which nucleoplasmin was purified. This rate is sufficient to account for the rate of nucleosome assembly required during the early development of Xenopus embryos.