The histone chaperone NAP1 is a carrier of histones during nuclear import, nucleosome assembly, and chromatin remodeling. Analytical ultracentrifugation was used to determine the association states of NAP1 alone and in complexes with core histones. In addition, the concentration dependence of the association was quantified by determining the equilibrium dissociation constant between different NAP1 species. At physiological protein and salt concentrations the prevalent species were the NAP1 dimer and octamer. These were also the association states found to interact with histones in a stoichiometry of one NAP1 monomer per histone. Based on these results a model for a cell cycle-dependent shift of the NAP1 dimer-octamer equilibrium is proposed that reflects different biological functions of NAP1.