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The first step in the formation of the nucleosome is commonly assumed to be the deposition of a histone H3−H4 heterotetramer onto DNA. Antisilencing function 1 (ASF1) is a major histone H3−H4 chaperone that deposits histones H3 and H4 onto DNA. With a goal of understanding the mechanism of deposition of histones H3 and H4 onto DNA, we have determined the stoichiometry of the Asf1−H3−H4 complex. We have established that a single molecule of Asf1 binds to an H3−H4 heterodimer using gel filtration, amino acid, reversed-phase chromatography, and analytical ultracentrifugation analyses. We demonstrate that Asf1 blocks formation of the H3−H4 heterotetramer by a mechanism that likely involves occlusion of the H3−H3 dimerization interface.