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The ROCKS system of computer programs is available to carry out most of the calculations involved in data acquisition and structure determination by the isomorphous replacement and molecular replacement methods. ROCKS is designed to be general, easy to use, reliable and efficient. It now includes facilities for precession and oscillation film scanning, data reduction, Wilson statistics, structure-factor calculations, isomorphous-replacement phasing, atomic parameter least-squares refinement, Fourier, Patterson and difference-Fourier calculations, contouring and peak searching, local scaling, Patterson superpositions, product and R-factor translational searches, phase combination and phase refinement by electron density modification. It is available at present in versions for IBM series 360 and 370 computers and for DEC VAX computers.