The application of new methods of small-angle scattering data interpretation to a contrast variation study of the 50 S ribosomal subunit of Escherichia coli in solution is described. The experimental X-ray data from contrast variation with sucrose are analysed in terms of the basic functions in real space and the scattering curves from the volume inaccessible to sucrose and from the regions inside this volume occupied mainly by RNA and by proteins are obtained. From these curves models of the shape of the 50 S subunit and its RNA-rich core are evaluated. These two shapes are positioned so that their difference, which approximates the volume occupied by the proteins, produces a scattering curve which is in good agreement with the scattering from the protein moiety.