Chicken erythrocyte nucleosome core particles can be dissociated quantitatively into histones (H3, H4)2 bound to 146 base pairs of DNA, and 2(H2A, H2B). Reconstitution of core particles from the two components produces an 85% yield of particles which neutron scattering studies show to be accurate stoichiometrically and indistinguishable from native core particles: the radii of gyration of the shape, the protein components and the DNA components of the particles are 4.02 nm, 3.3 nm and 4.95 nm respectively. The largest distance and most probable distance which can be drawn in the particles are 11.5 nm and 4.3 nm respectively. The molecular weight of the particles is identical to that of control 'native' core particles. All of these values, within limits of error, are the same as known values for 'native' core particles. These experiments confirm the essential role of histones H3 and H4 in the initial organisation of core-particle structure, make possible the manufacture of perfectly pure and homogeneous core-particle preparations and allow the 100% incorporation of labelled or modified histones. Neutron scattering studies of core particles at high contrast (in D2O and H2O) have been carried out over a range of ionic strengths and pH. No change in structure is detected down to pH 5.5 in 20 mM NaCl or down to ionic strength 2.0 mM at pH 7.