[en] A new form of three-dimensional crystals of the 50 S ribosomal subunits from Halobacterium marismortui has been obtained at 19⁰C, using polyethylene glycol in the presence of 1.2-1.7 M KCl in the crystallization mixture. The crystals diffract X-rays to 13 A and are stable in the synchrotron radiation beam for 4-8 h. Being aggregates of thin plates, the dimensions of only two unit cell edges, 174 x 181 A, with an angle of 95 deg. could so far be determined by both X-ray crystallography and electron microscopy. Attempts to produce thicker crystals by sophisticated seeding are in progress. (Auth.)